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Cards (750)
Section 1
(50 cards)
Source file:
A file containing the source code, having the file extension .java.
Reserved words
Words that have predefined meanings that cannot be changed
Binary operator:
An operator that has two operands.
Syntax
Rules for combining words into sentences
Syntax rules:
determine the validity of instructions in a programming language.
Source code:
The combination of the import statements and the program statements
Computer program:
A sequence of statements whose objective is to accomplish a task.
Arithemetic overflow
assigning a value to a variable that is outside of the ranges of values that the data type can represent
Cast operator:
Used for explicit type conversion
Objects
sent messages
Origin
the point (0,0) in a coordinate system
Mixed-mode arithmetic
expressions imvolving integers and floating-point values
Programming:
A process of planning and creating a program.
Name the 6 numeric data types
int, double, short, long, byte, and float
Semantic rules:
determine the meaning of instructions in a programming language.
Type casting
allows one data type to be explicitly converted to another
Operator precedence rules:
Determine the order in which operations are performed to evaluate an expression.
Data type:
A set of values together with a set of operations.
Primitive data types
numbers, characters, and Booleans. combined in expression with operators
Programming language:
A set of rules, symbols, and special words.
Run-time error
An error that occurs when a program asks the computer to do something that it considers illegal, such as dividing by 0
Variable declaration statement
declares the identifier and data type for a variable
What does /* suggest
a comment
Semantics
Rules for interpreting the meaning of statements
Why is there a semicolon (;) at the end of each statement?
So Java knows that it is the end of a statement.
Unary operator:
An operator that has only one operand.
Screen coordinate system
A coordinate system used by most programming languages in which the origin is in the upper-left corner of the screen, window, or panel, and the y values increase toward the bottom of the drawing area
Implicit type coercion:
When a value of one data type is automatically changed to another data type.
Virus
a program that can enter a computer and perhaps destroy information
String concatenation
append a string or value to another string
Identifier:
A Java identifier consists of letters, digits, the underscore character ( _), and the dollar sign ($), and must begin with a letter, underscore, or the dollar sign.
Pseudocode
A stylized half-English, half-code language written in English but suggestion Java code
How do you print something in the output?
System.out.print("Something to print"); or System.out.println("Something to print with a line (LiNe)");
Coordinate system
a grid that allows a programmer to specify positions of points in a plane or of pixels on a computer screen
Literals
items whose values do not change, like the number 5.0 or the string "Java"
Escape character (\)
used in codes to represent characters that cannot be directly typed into a program
Assignment operator:
The = (the equals sign). Assigns a value to an identifier.
Variable:
A memory location whose content may change during program execution.
Comments
explanatory sentences inserted in a program in such a manner that the compiler ignores them
Constants
variables whose values cannot change
Logic error
an error such that a program runs, but unexpected results are produced. Also referred to as a design error
Mixed expression:
An expression that has operands of different data types.
Named constant:
A memory location whose content is not allowed to change during program execution.
Exception
an abnormal state or error that occurs during runtime and is signaled by the operating system
console.nextDouble():
retrieves that floating-point number, if the value of this expression is that floating-point number.
Graphics context
in java, an object associated with a component where the images for that component are drawn
Token:
The smallest individual unit of a program written in any programming language.
Arithmetic expressions
a sequence of operands and operators that computes a value
Package
a group of related classes in a named directory
Java vocabulary
Set of all words and symbols in the language
Section 2
(50 cards)
Application
A program that runs locally, on your own computer.
Describe 2 classes can have.
1. Is a - inheritance 2. Has a - composition / aggregation V47
What are 6 assignment operators?
= += -= *= /= %= ----------------- v20
What is erasure?
TODO v62
Access Control
Public or Private; Determines how other objects made from other classes can use the variable, or if at all.
What is a 'set' in Java?
A grouping like a LIST but all the items are unique. There are no duplicates.
What does the 'new' key word do here when creating an instance? Printer myPrinter = new Printer();
Calls the default constructor on the Printer class.
Name to things about strings in Java.
1. Treated like a primitive type (can be assigned w/o the New keyword) but it's an object 2. They're immutable (can't be changed). If you give a string a different value, it create an new string in memory. Java Fundamentals - v22
In OOP, describe is-a or has-relationships?
Classes in OOP, are related to each other & they're going to be related through an "IS-A" relationship or "HAS-A" relationship. > IS-A relationship result from inheritance > HAS-A relationship result from composition This comes from the need to reuse code. v47
How do you create variables?
(A kind of variable) (name of the variable) = (value of variable); For example, String var = "Hello, World";
How do you make a Hello World program?
class helloWorld { public static main (String[] args) { String hw = "Hello, World!"; System.out.println(hw); } }
How do you create a method?
public void (method name)((arguments)) { } For example, public void greetYou(String greeting) { //Your program here! }
1. Create an Array called 'number' w/ 3 slots 2. Write out the 2nd number
int[] numbers = new int[3];
Create an IllegalAurgumentException (with some error message) in a method & on the calling method, add a try / catch block to display your error.
TODO - video on Excepions -->Try / Catch
How do you make a main method?
public static main(String[] args) { //Your program goes here! }
What is a main method for?
For letting your program run.
How do you make loops?
for (int i; i < 10; i++) { //Program }
Create an example of inheritance and using a constructor by writing a method that print the name passed into the constructor. In the super class, use the keyword 'protected'
TODO v48
Create an example of composition. Create a car class that HAS a gas tank. After the car runs 200miles, make it request to add gas.
TODO inspired by v49 (paper tray example)
Create an example using 'wild cards'
TODO v61
What are variables?
Things that store information.
Name 3 reasons why favor composition over inheritance
1. Almost all inheritance can be rewritten 2. Less about modeling the real world and more about modeling interactions in a system and roles and responsibilities 3. Inheritance is difficult to maintain V52
Applet
A program that runs on a web page.
Given: String firstName = "reynald"; Writhe the line of code to out the index of letter 'a'
System.out.println(firstName.indexOf('e')); v23
What's a method?
A mini-program that is referred to by the program and others.
What is this "x+=3" the equivalent of?
x = x + 3;
What are checked & unchecked exceptions.
1) Checked Exception is required to be handled by compile time while Unchecked Exception doesn't. 2) Checked Exception is direct sub-Class of Exception while Unchecked Exception are of RuntimeException.
Create an example of changing inheritance to composition by using interfaces. See exampled in v54.
TODO
What's a list in Java?
A grouping that is ordered, sequential & can have duplicates.
Create a generic method as in v59 and use bounded types (v60)
TODO
Exercise: 1. Create a printer class w/ a modelNumber property & a method to print that model #. 2. From you main method, assign a value to the modelNumber & invoke the Print() method.
Exercise. v(32)
What's the purpose of the Finally Block
It's useful to force code to execute whether or not an exception is thrown. It's useful for IO operations to close files.
What's the equivalent of a dictionary in Java? Please create an example.
Map - Look in module about collections.
Array
A group of related variables that share the same type and are being arranged.
Create an example of Polymorphism that reflect v51
TODO
What is a 'Queue' in Java?
TODO: Create a queue
What is a primitive type in Java?
In java, we have primitive types & objects. Primitive types hold the actual value in memory. Object hold references in memory.
How do you make a Switch statement?
switch (var) { case 1://if var = 1 //What happens break; case 2 ://if var = 2 //What happens break; default://Otherwise //What happens break; }
How do you make if...else statements?
if (arg) { } else { } if (arg) { } if (arg) { } else if (arg) { } else { } For example if (e < 10) { //What happens if e < 10 } else { //What happens otherwise } Switch statements are better for if...else if...else.
What are bounded types in Java?
Allows you to restrict the types that can be used in generics based on a class or interface. v60
How do you make a comment?
//Comment /*Multi-Line Comment*/ /**JavaDoc Comment *Describes Program or *Method **/
1. Define a variable that = to ten. 2. Create a while loop that prints you name tens time.
int ten = 10; while(copies > ten) { System.out.println("Reynald"); copies --; }
Name 9 primitive types
BSILF DB CS byte short int long float double boolean char String
appletviewer
A tool included in the JDK that's supported in NetBeans, which will test applets.
Argument
Extra information sent to a program.
How do you start a program?
class (name of program) { //Your program goes here!! }
Argument Storage
An Array.
Given this method below, implements 'continue' or 'break' statement if color is 'green' public void printColors() { String[] colors = new String[] {"red","blue", "green","yellow"}; for (String c : colors) { System.out.println(c); } }
public void printColors() { String[] colors = new String[] {"red","blue", "green","yellow"}; for (String c : colors) { if("green".equals(c)) continue; System.out.println(c); } }
Create a generic class as in v58
TODO
Create an array called 'colors' and add to it :"red","blue", "Green" 2. 1. Create a foreach loop the writes out the contents of the arra
1. String[] colors = new String[] {"red","blue", "Green"}; 2. for (String c : colors){ System.out.println(c); } v42
Section 3
(50 cards)
Object
A way of organizing a program so that it has everything it needs to accomplish a task.
Methods
Part of an Object's behavior.
Differences in String
S is Capitalized. Type of Object.
Unboxing
Casts an object to the corresponding simple value.
Multi-thread
A way for the computer to do more than one thing at the same time.
Autoboxing
Casts a simple variable value to the corresponding class.
Inheritance
Enables one object to inherit behavior and attributes from another object.
Method
A group of Attributes and Behaviors.
main()
Block statement statement in which all of the program's work gets done.
Attribute
The information that describe the object and show how it is different than other objects.
Constants
Variables that do not change in value; typed in all-caps.
Thread
Each part of a program which takes care of a different task.
Subclass
A class that inherits from another class.
Casting
Converting information from one form to another.
Objects
Programs that you create. These can be thought of as physical items that exist in the real world.
Class
A master copy of the object that determines the attributes and behavior an object should have.
Source
Information in it's original form.
Concatenating
Joining one string to another string. Also known as pasting.
Boolean Values
Type of variable that cannot be used in any Casting.
Three types of Loops
For, While, and Do-While.
Attributes and Behaviors
An object contains these two things.
Class Statement
The way you give your computer program a name.
Single '
Quotation type used for character values.
Do While Execution
This loop will always execute at least once, even if the conditions are not met.
Variable
A storage place that can hold information, which may change.
Double "
Quotation type used for string values.
Engaging in OOP
Breaking down computer programs in the same way a pie chart is broken down.
Ternary Operator
Used to assign a value or display a value based on a condition.
Comma
Used to separate things within a section.
Destination
The converted version of the source in a new form.
Superclass
A class that is inherited from.
Reason Brackets are missing
Not required for single statement IF statements or Loops.
Public int
Makes it possible to modify the variable from another program that is using the object.
char
Any character. Surrounded by single quotation marks.
OOP Program
A group of objects that work together to get something done.
String
A line of text that can include letters, numbers, punctuation, and other characters.
Do While Loop
Tests the condition at the end of each repetition of a section of a program.
While Loop
Tests the condition at the beginning of each repetition of a section of a program.
Semicolon
Used to separate sections.
Behavior
What an object does.
Method
A way to accomplish a task in a Java program.
Statement
An instruction you give a computer.
Element
An item in an Array.
Loop
This causes a computer program to return to the same place more than once.
Iteration
A single trip through a loop.
Programs
A class that can be used as a template for the creation of new objects.
Platform Independent
A program that does not have to be written for a specific operating system.
For Loop
Repeats a section of a program a fixed amount of times.
String
A collection of characters. Surrounded by double quotation marks.
Iterator
The counter variable used to control a loop.
Section 4
(50 cards)
public class{
Opens and names your program
Objects
belong to classes
System.out.print(" ");
Prints a string of text
Accessor
Extracts information from an object without changing it. Usually prefixed with 'get'.
Object references
stored in Object variables and denotes the memory location of an Object
println
Starts a new line after displaying the text.
Identifiers may contain ...
letters, digits and the underscore character
Package
Java classes grouped together in order to be imported by other programs
Assignment
variableName = value;
API documentation
lists the classes and methods of the Java library
Object reference
a value that denotes the location of the object in memory
Identifier
name of a variable, method or class
int
a primitive data type, integer
New operator
An operator that constructs new objects
Class Definition
accessSpecifier class ClassName { constructors methods fields }
Instance Field Declaration
accessSpecifier class ClassName { accessSpecifer fieldType fieldName; }
public static void main(String[] args){
Line 2 of program, opens programming block
length() method
method which counts the number of characters in a String
Method Definition
accessSpecifier returnType methodName(parameterType parameterName, . . .) { method body }
Method call
object.methodName(parameters);
Variable definition
typeName variableName = value;
toUpperCase
Method which converts any String to uppercase equivalent by creating a new String object
Cast
(typeName) expression
Declare a variable
int n;
trim() method
method which removes leading and trailing spaces from a String
Applet
Graphical Java program that runs in a web browser or viewer. To run one must have an HTML file with an applet tag.
Constant definition
final typeName variableName = expression;
Importing a Class from a Package
import packageName.ClassName;
Method
a sequence of instructions that accesses the data of an object
long totalMilliseconds = System.currentTimeMillis();
Sets total number of milliseconds since midnight, January 1, 1970, as a variable
primitive
data such as numbers and characters, NOT objects
The RGB values of Color.RED
255, 0, 0
overloaded
a class having more than one method with the same name
Initialize a variable
n = 5;
The return Statement
return expression;
Declare & Initialize a variable
int n = 5;
JFrame
used to show a frame(window)
Expressions
Statements that involve a mathematical equation and produce a result.
-
operator for subtraction
Static method call
ClassName.methodName(parameters);
Variables
Used to store values that can be used repeatedly in a class
import java.util.Scanner;
Import the ability to use a scanner from database
Constructor Definition
accessSpecifier ClassName(parameterType paramterName, ...) { constuctor body }
*
operator for multiplication
Assignment operator
an operator (=) that changes the value of the variable to the left of the operator
Object Construction
ClassName objectName = new ClassName(parameters);
Double var = input.nextDouble();
Establishes value inputed into scanner as a variable
Mutator
Method that alters the attributes of an object.
Public Interface
specifies what you can do with its objects
Scanner input = new Scanner(System.in);
Adds scanner to program
Section 5
(50 cards)
String: get last index of substring
str.lastIndexOf(char); // returns -1 if doesn't exist
String: concatenate two strings
str1.concat(str2); str1 + str2;
LinkedList: peek methods
list.peek(); //peeks head list.peekLast(); // peeks last element
HashMap: insert a key string and a value string
myMap.put("key","value");
ArrayList: check if list contains item
myArr.contains(item);
||
"Or" operator
&
Bitwise And operator
The if Statement
if (condition) statement; else statement;
The while Statement
while (condition) statement;
LinkedList: iterate through list of strings and print each one
Iterator<String> iter = list.iterator(); while(iter.hasNext()){ System.out.println(iter.next()); } don't forget: iterators have a remove() function!
LinkedList<String>: Instantiation
LinkedList<String> myList = new LinkedList<String>();
ArrayList: replace item at index i with new item
myArr.set(i, newItem);
What is a Vector?
Similar to ArrayList, but synchronized
Array Element Access
arrayReference[index]
String: get first index of substring
str.indexOf(char); // returns -1 if doesn't exist
~
Bitwise Not operator
HashMap: get a Set of the keys
myMap.keySet();
The "for each" loop
for (Type variable: collection) statement;
ArrayList: add "hi" to ArrayList at index i
myArr.add(i,"hi");
HashMap<String, String>: Instantiation
HashMap<String, String> myMap = new HashMap<String,String>();
ArrayList: delete item at index i
myArr.remove(i);
String: get char at index i
str.charAt(i);
ArrayList: get size
myArr.size();
HashMap: get value string from key string
myMap.get("key");
LinkedList: remove methods
same as arraylist remove methods, but also: remove(); // removes first item from list removeLast(); //removes last item from list
Enum: Instantiate Enum for days of the week
public enum Day{ Monday(0),Tuesday(1), Wednesday(2), Thursday(3),Friday(4),Saturday(5),Sunday(6); private int value; private Day(int v){value = v;} public int getValue(){return this.value;} } //values() returns array of all constants defined in enum // valueOf() returns Enum with name matching to String passed to valueOf // Day.name() returns string name of enum
string Array: Instantiation
String[] myArr = new String[numberOfItems]; String[] myArr = {"a","b","c"};
LinkedList: add item to beginning of list
-addFirst(item);
HashMap: see if map contains a key
myMap.contains("key");
&&
"And" operator
ArrayList: get first index of item
myArr.indexOf(item);
ArrayList<String>: Instantiation
ArrayList <String> myArr = new ArrayList<String>();
{}
Defines a block of code
!=
Boolean not equal to
HashMap: get a Collection of the values
myMap.values();
ArrayList: add "hi"
myArr.add("hi");
()
used to surround perimeters
Array Construction
new tymeName[length];
[]
Declares arrays
Block statement
{ statement; statement; }
|
Bitwise Or operator
String: remove whitespace
str.trim();
ArrayList: get last index of item
myArr.lastIndexOf(item);
HashMap<String,String>: iterate and print keys/values
for (String key : loans.keySet()) { System.out.println("key: " + key + " value: " + loans.get(key)); }
String: get substring "He" from "Hello"
str.subString(0,2); // begin index is inclusive, end index is exclusive
Array: convert from Collection strs to Array
String[] arr = new String[strs.size()]; strs.toArray(arr);
String: compare two strings
str1.equals(str2)
ArrayList: grab item at index i
myArr.get(i);
//
comment
The for Statement
for (initialization; condition; update) statement;
Section 6
(50 cards)
--
Decrement by one
Method that returns.
Define the keyword "return"
byte
8 bit integer
for( insert argument here) { insert code here }
For loop
;
End of a statement
*=
Multiply and assign operator
+
add operator
Method that does not return.
Define the keyword "void"
i
What is the standard convention for un-named variables?
=
Assignment
<
Less than
!
Boolean not
int i = 10; while(i > 5){ i = i + 5; System.out.print(i); }
Write an endless while loop where i=10
>=
Greater than or equal to
%
Take remainder operator
Abstract is assigned when you want to prevent an instance of a class from being created.
Define the keyword "abstract"
/=
divide and assign operator
short
16 bit integer
==
Boolean equals
int
32 bit integer
Null means nothing. It's a place holder.
Define "Null"
-
Subtraction operator
<=
Less than or equal to
if(i=10){ //do stuff }
Write an if then statement where i=10
%=
Take remainder and assign operator
.
separates package names / adds folders
Used to catch exceptions and tell the application to do something else.
Define the keyword "catch"
int i = 10; do{ i = i + 5; System.out.print(i); } while (i > 5);
Write an endless do loop where i=10
-=
subtract and assign operator
Only able to be modified inside the class.
Define the keyword "private"
boolean
true or false
if (i=10) { //do something } else { //do something else }
Write an if then else statement where i=10
Able to be modified by the class, package, subclass, and world.
Define the keyword "public"
while(insert argument here) { insert code here }
While loop
Does not change. Cannot make an instance of.
Define the keyword "static"
++
Increment by one
long
64 bit integer
public static void main(String args[])
Main Method
double
64 bit real number
for(i=10; i > 5; i++){ System.out.println(i); }
Write an endless for loop where i=10
*
Multiplication operator
Able to be modified by the class, package, and subclass. But not the world.
Define the keyword "protected"
used to implement an interface.
Define the keyword "implements"
+=
Add and assign operator
float
32 bit real number
/
Division operator
used to create a subclass for an existing class.
Define the keyword "extends"
char
A single character
do{ code here } while ( argument here);
Do loop
>
Greater than
Section 7
(50 cards)
for (initialization; condition; update) { statements }
terminating loop
Math.sqrt(x)
square root of x
System.out.print
prints statement
array.length
gives the length of the array (no parentheses!)
return volume;
return statement that returns the result of a method and terminates the method call
values[i] = 0;
access element of array at position i (count starts from 0)
Math.exp(x)
e^x
string Var = "Hello"
variable Var = Hello (*double quotes!)
++/--
increment/decrement; add 1 to/subtract 1 from the variable
if (string1.equals(string2))...
compares equality of 2 strings
char newChar = (char)(mychr + x)
shift character by x numbers (use ASCII table values)
/** Computes the volume of a cube. @param sideLength the side length of the cube @return the volume */
structure for commenting before methods (javadoc)
/* */
long comments
str.substring(start, end)
returns the string starting at position (start) and ending at position (end-1) in the given string
str.equals("test")
(boolean) returns true if the string "test" matches string str, false otherwise
while (condition) { statements }
statements = body of while loop; braces not needed for a single statement
double blah = (int) (blah + 3)
typecast; change variable type
str.length()
returns the number of characters in the given string (int)
string1.compareTo (string2) < 0
The compareTo method compares strings in lexicographic order.
java SentinelDemo < numbers.txt
redirects input
do { i++; //more code }while (i<max)
do/while loop
double[] values = new double[10]; double[] moreValues = {32, 54, 67.5};
array declaration and initialization
Math.sin(x)
sine of x (x in radians)
in.hasNextDouble()
checks for numerical input; false if the next input is not a floating-point number
&&, ||, !
boolean operators; and, or, not
public static double cubeVolume (double sideLength)
header for method
str.charAt(pos)
returns the character (char) at position pos (int) in the given string
char answer = 'y'
variable answer = y (*single quotes!)
==,!=,>,<,>=,<=
relational operators; equal, not equal, greater than, less than, greater than/equal to, less than/equal to
if (condition) { statements1 } else { statements2 }
if statement (else not required in all cases)
Math.cos(x)
cosine of x
public static void boxString(String contents)
void methods return no value, but can produce output
final int VOLUME = 8
names constant integer
Math.tan(x)
tangent of x
int i = in.nextInt();
declares and inputs integer value
Math.pow(x,y)
x^y (x>0, or x=0 and y>0, or x<0 and y is an integer)
\
char that comes after is exempted from compiler (ie printing "")
Character.isDigit(ch)
statement (boolean), ch (char); allows you to check if ch is a digit or not
String s = in.nextLine();
declares and inputs a line (a string of characters)
System.out.Printf("%3d", i*j)
% /number of spaces taken up/ Integer, variable
double d = in.nextDouble();
declares and inputs a real value (floating point #)
import java.util.Scanner
imports Scanner library
Math.toRadians(x)
convert x degrees to radians (ie, returns x* p/180)
Math.toDegrees(x)
convert x radians to degrees (ie, returns x*180/p)
{ double volume = sideLength sideLength sideLength; return volume; }
body of method
System.out.println
prints statement and creates new line
Scanner in = new Scanner(System.in);
declares Scanner "in"
str.indexOf(text)
returns the position (int) of the first occurrence of string (text) in string str (output -1 if not found)
//comment
short comments
Character.isLetter(ch); Character.isUpperCase(ch); Character.isLowerCase(ch); Character.isWhiteSpace(ch);
statement (boolean), ch (char); allows you to check if ch is a letter/uppercase/lowercase/whitespace
Section 8
(50 cards)
Constants
Identifiers that always have the same value.
new type [nRows] [nCols]
Create a 2D array
Class
The data type of an object
{ }
(Curly braces). Marks the beginning and end of a unit, including a class or method description.
Void
When a method does not return a value.
Instantiation
Creating an object.
d decimal integer f fixed floating-point e exponential floating-point (very large or very small values) g general floating-point s string
Format types (printf)
countryName = countryName.trim();
The trim method returns the string with all white space at the beginning and end removed.
new int [5] [8] new Pixel [numRows] [numCols]
Examples of creation of 2D arrays
throws FileNotFoundException
terminates main if FileNotFoundException occurs
boolean
A data type that represents a true or false value.
Attribute
A value an object stores internally.
Concatenation
Adding one string to another.
Boolean Literal
The words true and false.
Encapsulation
When each object protects its own information.
int value = matrix [3] [2]; Pixel pixel = pixels[r] [c];
Example: Accessing two elements
Assignment Statement
A statement assigning a value to a variable.
ArrayList<String> friends = new ArrayList<String>();
ArrayList<typeParameter> variableName = new ArrayList<typeParameter>(); typeParameter cannot be a primitive type
Integer.parseInt or Double.parseDouble
obtains the integer value of a string containing the digits
Arrays.copyOf(values, n)
The call Arrays.copyOf(values, n) allocates an array of length n, copies the frst n elements of values (or the entire values array if n > values.length) into it, and returns the new array.
friends.add (i, "Cindy"); String name = friends.get(i); friends.set(i, "Harry"); friends.remove(0);
adds element to arrayList at pos i (w/out i will add element to end); obtains element at i; redefines element at i as "Harry"; removes element
PrintWriter out = new PrintWriter("output.txt");
writes output to a file "output.txt"
Object
An instance of a class.
Method
A group of programming statements given a name.
Conversion
Changing one type of primitive data to another.
String input = in.next();
next() reads the next string; returns any charas that are not white space
int
A data type that represents whole numbers.
in.useDelimiter("[^A-Za-z]+");
restricts input to only letters, removes punctuation and numbers
Polymorphism
The idea that we can refer to objects of different but related types in the same way.
Operator Precedence
The rules that govern the order in which operations are done.
name [row] [col]
Access an element
double
A data type that represents decimals.
matrix[3] [2] = 8; pixels[r] [c] = aPixel;
Example of setting the value of elements
a.b()
Execute a method b which belongs to object a
Escape Sequence
A sequence used to represent special characters.
- Left alignment 0 show leading zeroes + show a plus sign for positive numbers ( enclose negative numbers in parentheses , show decimal separators ^ convert letters to uppercase
Format flags (printf)
Scanner in = new Scanner(new File("input.txt");
reads input from a file (also declared in same line)
b()
A method called b
name [row] [col] = value
Set the value of an element
for (typeName variable: array/collection) { sum = sum + variable; }
enhanced for loop for arrays; adds each element of array to sum (declared before loop)
String Literal
Anything within double quotation marks.
Inheritance
How one class can be created from another.
name.length
Get the number of rows
void
Means that the method does something but does not return any value at the end
;
Like a period in English. Marks the end of a statement or command
Primitive Data Types
Eight commonly used data types.
type[] [] name
Declare a 2D array
int [] [] matrix Pixel [] [] pixels
Examples of 2D array declarations
Arrays.toString(values) = [4,7,9...]
elements separated by commas with brackets
Variable
A location in memory used to hold a data value.
Section 9
(50 cards)
An open brace is always followed by a close
brace }
If the ArrayList object capacity is exceeded what will it automatically do?
"It will automaticallly resize" An ArrayList object will automatically resize if its capacity is exceeded.
What is the perferred method of storing datat in an array?
In most cases, the ArrayList is the preferred method of storing data in an array.
What escape sequence makes the backspace character?
\b
How could you define a real number named numero and initialize it to 867.5?
double numero = 867.5
______
______ is the escape sequence for
new line
...
An ArrayList object can have its internal capacity adjusted manually to improve efficiency.
...
Multi-dimensional arrays can be initialized by placing the values that will populate the array in curly brackets. Each dimension is represented by a nested set of curly brackets. Example: int[ ][ ][ ] threeDArray = {{{1,2}, {3,4}},{{5,6},{7,8}},{{9,10},{11,12}}}
What System.out method moves the cursor to the next line?
System.out.println()
How many dimensions can standard Java arrays have?
Standard Java arrays can have one or many dimensions.
...
Multi-dimensional arrays do not have to have the same size sub arrays. Example: int[ ][ ] oddSizes = {{1,2,3,4},{3 },{6,7}}
What symbols are used to make comments in Java? (Both single and block comments)
// and / /
What is the ArrayList class a representation of?
The ArrayList class is an object-orientated representation of a standard Java array.
Can standard Java arrays be multi-dimensional?
Yes, standard Java arrays can be multi-dimensional; each set of square brackets represents a dimension.
What operator divides two numbers?
/
System.out.println("funny"); prints out
funny
Are there standard Java arrays built into the Java language?
Yeah, hell Yeah lol there standard Java arrays are built into the Java language.
_______\t________ is the escape sequence
tab
What are standard Java arrays and when must they be initialized?
Standard Java arrays are objects and must be initialized after they are declared.
A ________ is used to terminate all java
a semi colon ;
...
Standard Java arrays can be initialized by placing the values that will populate the array in curly brackets. Example: int[ ] myArray = {1,2,3,4}
What is the ASCII value of 'A'?
65
...
Multi-dimensional arrays can declared with the new operator by placing the size of each dimension in square brackets. Example: int[ ][ ][ ] threeDArray = new int[3][2][2]
...
An ArrayList object can have any element in it removed and will automatically move the other elements.
name[0].length
Get the number of columns
Never put a semi-colon before an open
brace{
What operator multiplies two numbers?
*
In Java, every program statement ends with what?
; (a semicolon)
System.out.println("fun\tny"); prints out
fun ny
What is the ASCII value of 'a'?
97
...
Multi-dimensional arrays do not have to declare the size of every dimension when initialized. Example: int[ ][ ][ ] array5 = new int[3][ ][ ]
What will automatically happen if you add elements at any index in the ArrayList object?
"It will automatically move the other elements" [Orginal below] An ArrayList object can have elements added at any index in the array and will automatically move the other elements.
System.out.println("abc"); prints out
abc
matrix[0].length pixels[0].length
Example: get the number of columns
What must every method and class have an open and closed of?
{ } (braces, or curly brackets)
What escape sequence makes the newline character?
What escape sequence makes one backslash character in a print or println statement?
\\
System.out.println("ab\\c"); prints out
ab\c
Work with ArrayList Objects and Their Methods
...
What can standard Java arrays store?
Standard Java arrays can store both objects and primitives.
matrix.length pixels.length
Example of getting the number of rows
Work with Java Arrays
Chapter 6 Objective 1
What package is the ArrayList part of?
" java.util package " The ArrayList class is part of the java.util package.
....
Standard Java arrays can be declared with square brackets after the type or variable name. Example: int[] myArray or int my Array[]
What escape sequence makes the tab character?
\t
System.out.println("ab\"\\c"); prints out
ab"\c
.....How would a three dimensional array be declared as? (How would it look like?)
A three dimensional array would be declared as int[ ][ ][ ] threeDArray.
How could you define an integer named num and initialize it to 394?
int num = 394;
How could you define a String variable named wrd and initialize it to the word cats?
String wrd = "cats";
...
Standard Java arrays can be initialized with the new operator and their size in square brackets after the type. Example: int[ ] myArray = new int[4]
Section 10
(50 cards)
semantics
define the rules for interpreting the meaning of statements
double
8 bytes, stores floating-point numbers
objects
second category of Java variables such as scanners and strings
long
4 bytes, stores long integer values
Define the Java related "object" and what is it interchangeable with?
Object and class define state (data_ and behavior (operations). Classes define an object's behavior.
Naming Methods?
Begins with public then is whatever data is going to be returned. ex. public String displayname()
Reasons to use "float"
127 digits after the decimal point
How to declare variables with the same data type on the same line?
Separate them with commas. ex. int number, salary, hoursWorked; (end with semi-colon)
.java
All Java file names have to have this extension.
Define how java classes are connected.
Loosely connected objects that interact with each other.
What does empty Parenthesis at the end of a method declaration mean?
That the data for the method will be put in later.
How do you assign data to a variable?
"=". ex. int number = 100
Example of variable declaration and explanation.
String firstName; Start with data type then variable name. Must end with the ";".
How to define a class?
public class "class name" (no extension)
statements
lines of code in java
Private variable declaration?
Only applies to that class. ex. private String first;
concatenation operator
allows strings to be combined in Java
Command to print to the console? (what letter is capitalized)
System.out.println("Hello");
short
2 bytes, stores integer values
Standard Starting, to start code block?
public static void main(String[] args) { //code goes here }
Alphabetic characters that can be stored as a numeric value?
char
syntax
the rules for combining words into sentences
Capital Letter
All java file names must have this at the beginning.
What could a object be compared to for similarity?
like base blocks that are interchangeable in how they can be used in other programs.
Insert a comment? On multiple lines?
double slash ex. //commented slash asterisk ex. /* one line two lines */ (its the back slash for both)
For numeric data types what declaration will always be used?
double
0 or more characters surrounded by double quotes?
String (always capital "S")
How to use sub-string function?
...
What must a class name always start with?
A capital letter then camel case with no spaces.
Surround all String text with?
Double Quotes
Reasons to use "double"
1023 digits after the decimal point
numeric data types
another name for primitive data types; most commonly int and double
programming vs natural languages
size, rigidity, literalness
What must variable names have/not have?
Have: Starts with a lower case letter. Not Have: after initial letter cannot have a symbol other than the underscore (no spaces).
int
4 bytes, stores integer values
True False data type
boolean
primitive data types
first category of Java variables such as numbers, characters, and Booleans
Set of print commands to print to the console on two different lines?
System.out.print("Hello"); //no "ln" System.out.print("World"); //Has the "ln"
Methods?
Are always public. They return data. Methods are made so that you can call on them so that you can use its functionality.
What is camel casing?
Each time a new word begins start it with a capital letter
Reasons to use "long"
Numbers greater then ±2 billion
What word is always capitalized in java syntax?
String
Reasons to use "int"
Numbers -2 billion to, 2 billion
vocabulary
the set of all of the words and symbols in a given language
4 Java Data Types?
Numeric, String, Character, Boolean.
String combination identifier
"+"
return function?
Returns and expression. ex. return first + " " + middle + " " + last;
Numeric Data Sub-Types
int, long, float, double
What non-numeric data type can have class operations preformed to it?
String
What must every line of code not followed by curly braces end in?
";"
Section 11
(50 cards)
remainder or modulus operator
%, fourth highest precedence, left to right association
grouping operator
( ), highest precedence
multiplication operator
*, fourth highest precedence, left to right association
logic errors
when the computer is unable to express an answer accruately
method selector operator
., second highest precedence, left to right association
A no-argument constructor has....
No parameters.
A parameter is a local variable initialized to....
the value of the corresponding argument. (Known as Call-by-value)
division operator
/, fourth highest precedence, left to right association
A class is a type whose values are ______
Objects
Constructor?
A special type of method that is called when an object is created to initialize variables.
Overloading the method?
Creating multiple definitions for the same method name, using different parameter types
unary plus operator
+, third highest precedence
Post Condition?
What will be true after a method is called
Private instance variables can be accessed from outside their class through ______ methods and changed with _________ methods
Accessor methods && Mutator methods
\t
indicates a tab character
assignment operator
=, lowest precedence, right to left association
literals
items in programs whose values do not change; restricted to the primitive data types and strings
variable declaration statement
when a program declares the type of a variable
A local variable disappears when...
a method invocation ends.
* If a variable is used as an argument...
ONLY the value of the the argument is plugged into the parameter, NOT the variable itself
Encapsulation?
Data and actions combined into a class whose implementation details are hidden
A _____________ is like a blank in a method definition...
Parameter
import x.y.z
x - overall name of package, y - name of package subsection, z - name of particular class in subsection
All objects in a class have the same _______ and same types of _______.
Methods && Instance Variables
syntax errors
when a syntax rule is violated
Parameters are filled in with _________________ when invoking a method.
Arguments
arithmetic expression
consists of operands and operators combined in a manner familiar from algebra
Helper methods should be
marked as private
escape character
used when including 'special characters' in string literals
If an instance variable or method is _____ then it cannot be directly referenced anyplace except in the definition of a method of the same class.
Private
float
4 bytes, stores less precise floating-point numbers
addition operator
+, fifth highest precedence, left to right association
An object is this type of variable
A reference variable
cast operator
(double),(int), third highest precedence, right to left association
The two main kinds of methods:
methods that return a value / void methods
T or F: The name of a local variable can be reused outside of the method in which it is declared ?
True
indicates a newline character
run-time errors
when a computer is asked to do something that is considered illegal
What is the method signature of public void setDate(int month, int day);?
setDate(int, int);
keywords
words in java that cannot be used as user-defined symbols
\\
escape character for backslash
instantiation operator
new, third highest precedence, right to left association
When defining a method, the _____ parameter is a name used for the calling object.
"this."
A variable declared in a method is said to be a ____________________.
Local variable
byte
1 byte, stores an integer
Objects have both _______ and _______.
Instance Variables && Methods
unary minus operator
-, third highest precedence
What is assumed to be true when a method is called
Precondition
Your classes should generally have both an _______ method and a _____ method.
equals && toString
subtraction operator
-, fifth highest precedence, left to right association
Section 12
(50 cards)
When you type something wrong and java doesn't compile (LANGUAGE)
Syntax error
blank.
Value
to convert data type
Casting
Prints x on the same line, then moves the cursor to the next line
println(x)
Value: true/false
boolean
-put data type you want in parenthesis before data you want to convert EX: int x = (int) 12.3; //cast double value of 12.3 as int, now has a value of 12
Casting Rules
-All must have one (except apps) -Follows the class, where execution begins -Must h ave { and }
Main method
When something goes wrong while the program is running. (Also known as exception)
Runtime error
-Must end with a ; (semicolon)
Statements
float input
nextFloat()
-64 bits Range: -big to big No rules
long
-All programs start with a ? -Must have { and } -Identifier (name) begins with a CAPITAL letter EX: public class Westlake
Class
What are the TWO types of data contained by classes?
Characteristics and Behavior
-Must start with a letter -Can contain letters, numbers, and underscore character -Cannot contain spaces -Cannot contain reserved (key) words
Identifiers (name) Rules
-8 bits Range: -128 to 127 No rules
byte
Naming standard for mutator method?
setXYZ();
Name the variable NOTE: starts with lower case, new word starts with capital letter (numDays)
Identity
-16 bits Range: -32,768 to 32,767 No rules
short
-Begins with a \ -Always comes in pairs
Escape sequences
New line
Prints x on the same line
print(x)
-Segments of code that perform some action -Always followed by parenthesis EX: System.out.println("abc") <--- print is the ?
Methods
-64 bits Range: -big to big
double
"
\"
Backspace
\b
Naming standard for accessor method?
getABC();
-short and char are not interchangeable -char is unsigned (only positive values), short has both positives and negatives
Mixing Data Types Rules
Integer value: 48
'0'
Stores a specific type of value
Primitive data types
\
\\
'
\'
-16 bits unsigned Range: 0 to 65535 -Holds one letter -Value enclosed in ' ' -Outputs letter unless addition is in parenthesis
char
During runtime, value assigned that exceeds positive bounds of a variable
Overflow error
Tab
\t
byte input
nextByte()
Stores (points to) the memory address of an object
Reference variable
Value: group of characters -Capitalize the S, enclose value in " "
String
Integer value: 97
'a'
double input
nextDouble()
// marks the single line ? / block ? /
Comments
-Cannot put a decimal into an integer -Integers cannot hold decimal values EX: int = double <---WRONG EX: double = int <--- CORRECT
Mixing Data Types Rules
-32 bits Range: -big to big -Must put an f at the end of the value EX: 98.6f; <--- f won't print
float
What kind of value and how much will be stored?
Data Type
short input
nextShort()
int input
nextInt()
-Cannot put a bigger box into a smaller box -Data type on RIGHT side of = has to be less than the type on the LEFT side EX: 64bits = 32bits <---CORRECT EX: 32bits = 64bits <--- WRONG
Mixing Data Types Rules
Integer value: 65
'A'
=
Assignment Operator
;
Semicolon
-32 bits Range: -2,147,483,648 to 2,147,483,647 No rules
int
Section 13
(50 cards)
Loop count to 10
for(count = 0; count<11: count++);
Method call/actual and formal for method name; No value, send parameters
variableName = methodName();
//
double slash; marks the beginning of a comment (a statement ignored by the computer)
Defining Parameters
...
Print statement
System.out.println("");
out
an object that provides a way to send output to the screen
( )
opening/closing parentheses; used in a method header
println
a method that prints characters to the screen and moves to the next line
String declaration
String name = "";
String comparing a to b
a.compareTo("b");
Method call/actual and formal for method name; Return value, no parameters
variableName = methodName(); public static int methodName():
" "
quotation marks; encloses a string of characters, such as a message that is to be printed on the screen
a method that prints characters to the screen and remains on the same line
Conditional operator
condition?truepath:falsepath; (hours<12 ? "AM" : "PM";)
New tab character
\t
public static void main(String [ ] args)
method header;
Method call/actual and formal for method name; No return value, no parameters
methodName(); public static void methodName():
Instance variables
Set to 0 a = 0; b = 0; c = 0;
{ }
opening/closing braces; encloses a group of statements, such as the contents of a class or a method
/t
horizontal tab; causes the curson to skip over to the next tab stop
public class _____
class header; indicates a publicly accessible class named "_____"
method
a group of one or more programming statemetnts that collectively has a name
\\
backslash; causes a backslash to be printed
Build a void method
public void main
Instantiating an object
class name object name = classname();
New line character
System.out.print( _____ );
prints the content in the parentheses ( _____) and remains on the same line
Logical Operators
&& ||
Keyboard integer input
keyboard.nextInt();
Print format statement to get 3.14 of pi
System.out.printf("%.2f", pi);
System.out.println( _____ );
prints the content in the parentheses ( _____ ) and moves to the next line
System
a class that holds objects/methods to perform system-level operations, like output
String input
next() nextLine()
static
One copy and everyone can see it
nextLine()
String of words
/b
causes the cursor to back up, or move left, one position
/** */
opening comments
Switch statement
switch (variable you're check against) { case label: code; break; default:
/r
return; causes the cursor to go to the beginning of the current line, not the next line
;
semicolon; marks the end of a complete programming statement
Emergency exit
Sytem.exit(value);
Method call/actual and formal for method name; Return value, no parameters
...
Constants declaration
public static final NAME = value;
Comparison operators: Equal to, Not equal to, greater than, greater than or equal to, less than, less than or equal to
== != > >= < <=
When an array is created, all elements are initialized with ___. A) zero B) array elements are not initialized when an array is created C) null D) a fixed value that depends on the element type
Ans: D Section Ref: Section 7.1 Arrays
Initialize scanner
Scanner keyboard= new Scanner (System.in);
next()
One word String
/n
newline; advances the curson to the next line for subsequent printing
Length method
name.length()
Build a return method
public return
Section 14
(50 cards)
Why can't Java methods change parameters of primitive type? A) Java methods can have no actual impact on parameters of any type. B) Parameters of primitive type are considered by Java methods to be local variables. C) Parameters of primitive type are immutable. D) Java methods cannot accept parameters of primitive type.
Ans: B Section Ref: Quality Tip 8.3
What is the purpose of this algorithm? int m = 0; for (BankAccount a : accounts) { if (a.getBalance() >= atLeast) m++; } A) Counting matches. B) Finding a value. C) Computing the sum. D) Locating an element.
Ans: A Section Ref: Section 7.6 Common Array Algorithms
Which of the following is considered by the text to be the most important consideration when designing a class? A) Each class should represent an appropriate mathematical concept. B) Each class should represent a single concept or object from the problem domain. C) Each class should represent no more than three specific concepts. D) Each class should represent multiple concepts only if they are closely related.
Ans:B Section Ref: 8.1
When you use a timer, you need to define a class that implements the ____ interface. A) TimerListener B) TimerActionListener C) ActionListener D) StartTimerListener
Ans: C Section Ref: 9.10
Wrapper objects can be used anywhere that objects are required instead of ____. A) primitive data types B) clone methods C) array lists D) generic classes
Ans: A Section Ref: Section 7.3 Wrappers and Auto-boxing
Which of the following statements about an inner class is true? A) An inner class may not be declared within a method of the enclosing scope. B) An inner class may only be declared within a method of the enclosing scope. C) An inner class can access variables from the enclosing scope only if they are passed as constructor or method parameters. D) The methods of an inner class can access variables declared in the enclosing scope.
Ans: D Section Ref: 9.8
What is the purpose of this algorithm? for (int i = 0; i < data.size(); i++) { data.set(i, i ); } A) Filling in initial values for an array. B) Counting matches in an array. C) Filling in initial values for an array list. D) Counting matches in an array list.
Ans: C Section Ref: Section 7.6 Common Array Algorithms
Which of the following represents the hierarchy of GUI text components in the hierarchy of Swing components? A) JTextField and JTextComponent are subclasses of JTextArea. B) JTextArea and JTextComponent are subclasses of JTextField. C) JTextField and JTextArea are subclasses of JTextComponent. D) JTextField, JTextArea, and JTextComponent are subclasses of JPanel.
Ans: C Section Ref: 10.1
Which of the following is an event source? A) A JButton object. B) An event listener. C) An inner class. D) An event adapter.
Ans: A Section Ref: 9.7
Consider the following code snippet: final RectangleComponent component = new RectangleComponent(); MouseListener listener = new MousePressListener(); component.addMouseListener(listener); ______________ frame.add(component); frame.setSize(FRAME_WIDTH, FRAME_HEIGHT); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setVisible(true); Which of the following statements completes this code? A) private static final int FRAME_WIDTH = 300; B) private static final int FRAME_HEIGHT = 400; C) Frame frame = new Frame(); D) JFrame frame = new JFrame();
Ans: D
Which code fragment constructs an arrray list named players that is initialized to contain the strings "Player 1" and "Player 2"? A) ArrayList<String> players = new ArrayList<String>(); players.set(0, "Player 1"); players.set(1, "Player 2"); B) ArrayList<String> players = new ArrayList<String>(); players.set(1, "Player 1"); players.set(2, "Player 2"); C) ArrayList<String> players = { "Player 1", "Player 2" }; D) ArrayList<String> players = new ArrayList<String>(); players.add("Player 1"); players.add("Player 2");
Ans: D Section Ref: Section 7.2 Array Lists
Which of the following is not a reason to place classes into a package? A) to avoid name clashes (most important) B) to organize related classes C) to show authorship of source code for classes D) to make it easier to include a set of frequently used but unrelated classes
Ans: D Section Ref: 8.9
Which of the following statements generally describes the scope of a variable? A) The ability of a variable to access other variables in the same program. B) The amount of data in bytes that the variable can contain. C) The range of data types that the variable can legally be cast into. D) The region of a program in which the variable can be accessed.
Ans: D Section Ref: 8.8
Which of the following statements describes an assertion? A) A logical condition in a program that you believe to be true. B) A guarantee that the object is in a certain state after the method call is completed. C) A requirement that the caller of a method must meet. D) A set of criteria defined by the caller of a method.
Ans: A Section Ref: 8.5
Mutator methods exhibit which of the following types of side effect? A) Modification of the implicit parameter. B) Modification of an explicit parameter. C) Production of printed output. D) Acceptance of text input.
Ans: A Section Ref: 8.4
A method that has no implementation is called a/an ____ method. A) interface B) implementation C) overloaded D) abstract
Ans: D Section Ref: 9.1
Which of the following code statements creates a graphical button which has "Calculate" as its label ? A) Button JButton = new Button("Calculate"). B) button = new Button(JButton, "Calculate"). C) button = new JButton("Calculate"). D) JButton button = new JButton("Calculate").
Ans: D Section Ref: 9.9
To override a superclass method in a subclass, the subclass method ____. A) Must use a different method name. B) Must use the same method name and the same parameter types. C) Must use a different method name and the same parameter types. D) Must use a different method name and different parameter types.
Ans: B Section Ref: 10.3
To create a subclass, use the ____ keyword. A) inherits B) implements C) interface D) extends
Ans: D Section Ref: 10.2
Which of the following statements about abstract methods is true? A) An abstract method has a name, parameters, and a return type, but no code in the body of the method. B) An abstract method has parameters, a return type, and code in its body, but has no defined name. C) An abstract method has a name, a return type, and code in its body, but has no parameters. D) An abstract method has only a name and a return type, but no parameters or code in its body.
Ans: A Section Ref: 9.1
What is the purpose of this algorithm? double total = 0; for (double element : data) { total = total + element; } A) Computing the sum. B) Finding a value. C) Locating an element. D) Counting matches.
Ans: A Section Ref: Section 7.6 Common Array Algorithms
Which class manages a sequence of objects whose size can change? A) Array B) Object C) Sequence D) ArrayList
Ans: D Section Ref: Section 7.2 Array Lists
Fill in the if condition that will only access the array when the index variable i is within the legal bounds. if (____________________) data[i] = value; A) 0 <= i < data.length() B) 0 <= i && i < data.length C) 0 <= i < data.length D) 0 <= i && i < data.length()
Ans: B Section Ref: Section 7.1 Arrays
Under which of the following conditions can you have local variables with identical names? A) If their scopes are nested. B) If they are of different data types. C) If their scopes do not overlap. D) If they both reference the same object.
Ans: C Section Ref: 8.8
Which of the following is true regarding subclasses? A) A subclass inherits methods from its superclass but not instance variables. B) A subclass inherits instance variables from its superclass but not methods. C) A subclass inherits methods and instance variables from its superclass. D) A subclass does not inherit methods or instance variables from its superclass.
Ans: C Section Ref: 10.2
Why is a static variable also referred to as a class variable? A) There is a single copy available to all objects of the class. B) It is encapsulated within the class. C) Each class has one and only one static variable. D) It is stored in the separate class area of each object.
Ans: A Section Ref: 8.7
Which of the following types of side effects potentially violates the rule of minimizing the coupling of classes? A) Standard output. B) Modification of implicit parameters. C) Modification of explicit parameters. D) Modification of both implicit and explicit parameters.
Ans: A Section Ref: 8.4
Consider the following code snippet: public class Demo { public static void main(String[] args) { Point[] p = new Point[4]; p[0] = new Colored3DPoint(4, 4, 4, Color.BLACK); p[1] = new ThreeDimensionalPoint(2, 2, 2); p[2] = new ColoredPoint(3, 3, Color.RED); p[3] = new Point(4, 4); for (int i = 0; i < p.length; i++) { String s = p[i].toString(); System.out.println("p[" + i + "] : " + s); } return; } } This code is an example of ____. A) overloading B) callback C) early binding D) polymorphism
Ans: D Section Ref: 9.3
General Java variable naming conventions would suggest that a variable named NICKEL_VALUE would most probably be declared using which of the following combinations of modifiers? A) public void final B) public static final double C) private static double D) private static
Ans: B Section Ref: 8.2
A class that represents the most general entity in an inheritance hierarchy is called a/an ____. A) Default class. B) Superclass. C) Subclass. D) Inheritance class.
Ans: B Section Ref: 10.1
When designing a hierarchy of classes, features and behaviors that are common to all classes are placed in ____. A) every class in the hierarchy B) the superclass C) a single subclass D) all subclasses
Ans: B Section Ref: 10.1
How do you specify what the program should do when the user clicks a button? A) Specify the actions to take in a class that implements the ButtonListener interface. B) Specify the actions to take in a class that implements the ButtonEvent interface . C) Specify the actions to take in a class that implements the ActionListener interface. D) Specify the actions to take in a class that implements the EventListener interface.
Ans: C Section Ref: 9.9
Which of the following statements will compile without error? A) public interface AccountListener() { void actionPerformed(AccountEvent event); } B) public interface AccountListener() { void actionPerformed(AccountEvent); } C) public interface AccountListener { void actionPerformed(AccountEvent event); } D) public abstract AccountListener { void actionPerformed(AccountEvent event); }
Ans: C Section Ref: 9.7
Which of the following describes an immutable class? A) A class that has no accessor or mutator methods. B) A class that has no accessor methods, but does have mutator methods. C) A class that has accessor methods, but does not have mutator methods. D) A class that has both accessor and mutator methods.
Ans: C Section Ref: 8.3
____ are generated when the user presses a key, clicks a button, or selects a menu item. A) Listeners B) Interfaces. C) Events. D) Errors.
Ans: C Section Ref: 9.7
What is the purpose of this algorithm? for (BankAccount a : accounts) { if (a.getAccountNumber() == accountNumber) return a; } return null; A) Finding a value. B) Counting matches. C) Computing the sum. D) Inserting an element.
Ans: A Section Ref: Section 7.6 Common Array Algorithms
What is the type of a variable that references an array list of strings? A) ArrayList[String]() B) ArrayList<String>() C) ArrayList<String> D) ArrayList[String]
Ans: C Section Ref: Section 7.2 Array Lists
What is the type of a variable that references an array of integers? A) int[] B) integer() C) integer[] D) int()
Ans: A Section Ref: Section 7.1 Arrays
A static method can have which of the following types of parameters? A) Only implicit parameters. B) Only explicit parameters. C) Both implicit and explicit parameters. D) A static method cannot have parameters.
Ans: B Section Ref: 8.6
Which of the following statements describes a precondition? A) A logical condition in a program that you believe to be true. B) A guarantee that the object is in a certain state after the method call is completed. C) A requirement that the caller of a method must meet. D) A set of criteria defined by the caller of a method.
Ans: C Section Ref: 8.5
You access array list elements with an integer index, using the __________. A) () operator B) [] operator C) get method D) element method
Ans: C Section Ref: Section 7.2 Array Lists
What is the error in this array code fragment? double[] data; data[0] = 15.25; A) The array referenced by data has not been initialized. B) A two-dimensional array is required. C) An out-of-bounds error occurs. D) A cast is required.
Ans: A Section Ref: Section 7.1 Arrays
Consider the following code snippet: class MouseClickedListener implements ActionListener { public void mouseClicked(MouseEvent event) { int x = event.getX(); int y = event.getY(); component.moveTo(x,y); } } What is wrong with this code? A) The mouseClicked method cannot access the x and y coordinates of the mouse. B) repaint() method was not called. C) The class has implemented the wrong interface. D) There is nothing wrong with this code.
Ans: C Section Ref: 9.11
The wrapper class for the primitive type double is ____________________. A) There is no wrapper class. B) Double C) doubleObject D) Doub
Ans: B Section Ref: Section 7.3 Wrappers and Auto-boxing
Which of the following statements about a Java interface is NOT true? A) A Java interface defines a set of methods that are required. B) A Java interface must contain more than one method. C) A Java interface specifies behavior that a class will implement. D) All methods in a Java interface must be abstract.
Ans: B Section Ref: 9.1
Which of the following questions should you ask yourself in order to determine if you have named your class properly? A) Does the class name contain 8 or fewer characters? B) Is the class name a verb? C) Can I visualize an object of the class? D) Does the class name describe the tasks that this class will accomplish?
Ans: C Section Ref: 8.1
You have a class which extends the JComponent class. In this class you have created a painted graphical object. Your code will change the data in the graphical object. What additional code is needed to ensure that the graphical object will be updated with the changed data? A) You must call the component object's paintComponent() method. B) You must call the component object's repaint() method. C) You do not have to do anything - the component object will be automatically repainted when its data changes. D) You must use a Timer object to cause the component object to be updated.
Ans: B Section Ref: Common Error 9.6
Consider the following code snippet: public interface Sizable { int LARGE_CHANGE = 100; int SMALL_CHANGE = 20; void changeSize(); } Which of the following statements is true? A) LARGE_CHANGE and SMALL_CHANGE are automatically public static final. B) LARGE_CHANGE and SMALL_CHANGE are instance variables C) LARGE_CHANGE and SMALL_CHANGE must be defined with the keywords private static final. D) LARGE_CHANGE and SMALL_CHANGE must be defined with the keywords public static final.
Ans: A Section Ref: Special Topic 9.1
Which of the following is a good indicator that a class is overreaching and trying to accomplish too much? A) The class has more constants than methods B) The public interface refers to multiple concepts C) The public interface exposes private features D) The class is both cohesive and dependent.
Ans: B Section Ref: 8.2
Rewrite the following traditional for loop header as an enhanced for loop, assuming that ArrayList<String> names has been initialized. for (int i = 0; i < names.size(); i++) { // process name } A) for (String s : names) B) for (int i = 0; i < names.size(); i++) C) for (s : names) D) for (names : String s)
Ans: A Section Ref: Section 7.4 The Enhanced for Loop
Section 15
(50 cards)
Exception
This class indicates issues that an application may want to catch. A subclass of the class Throwable.
print (return)
System.out.println();
java.util.*;
To work with collections framework, event model, and date/time facilities.
double
Used to declare and initialize a fraction interager. Ex: double canVolume = 0.335;
int
Used to declare and initialize a non fraction interager Ex: int canVolume = 5;
java.awt.*;
To paint basic graphics and images.
==
Used to equal a value
Consider the following code snippet: Vehicle aVehicle = new Auto(4,"gasoline"); String s = aVehicle.toString(); Assume that the Auto class inherits from the Vehicle class, and neither class has an implementation of the toString() method. Which of the following statements is correct? A) The toString() method of the Object class will be used when this code is executed. B) The toString() method of the String class will be used when this code is executed. C) This code will not compile because there is no toString() method in the Vehicle class. D) This code will not compile because there is no toString() method in the Auto class.
Ans: A Section Ref: 10.7.1
Literal
A value represented as an integer, floating point, or character value that can be stored in a variable.
In Java, every class declared without an extends clause automatically extends which class? A) this B) Object C) super D) root
Ans: B Section Ref: 10.7
/ /
Used to comment out a paragraph or longer
b
b
Which of the following statements about a superclass is true? A) An abstract method may be used in a superclass when there is no good default method for the superclass that will suit all subclasses' needs. B) An abstract method may be used in a superclass to ensure that the subclasses cannot change how the method executes. C) An abstract method may be used in a superclass when the method must have identical implementations in the subclasses. D) An abstract method may not be used in a superclass.
Ans: A Section Ref: Special Topic 10.1
declare a as an int
int a;
&&
Boolean operator meaning both sides have to be true. A Short circuiting logical operator.
b
b
Scope
Block of code in which a variable is in existence and may be used.
J2SE
Acronym for Java 2 Platform, Standard Edition.
java.lang.*;
To utilize the core java classes and interfaces.
declare a boolean variable, isAscending
boolean isAscending;
Relative Path
A path that is not anchored to the root of a drive. Its resolution depends upon the current path. Example ../usr/bin.
b
b
||
Boolean operator meaning only one side has to be true. A Short circuiting logical operator.
javax.swing.*;
To create lightwieght components for a GUI.
b
b
b
b
J2ME
Acronym for Java 2 Platform, Micro Edition.
A class that cannot be instantiated is called a/an ____. A) Abstract class. B) Anonymous class. C) Concrete class. D) Non-inheritable.
Ans: A Section Ref: Special Topic 10.1
Which of the following is true regarding inheritance? A) When creating a subclass, all methods of the superclass must be overridden. B) When creating a subclass, no methods of a superclass can be overridden. C) A superclass can force a programmer to override a method in any subclass it creates. D) A superclass cannot force a programmer to override a method in any subclass it creates.
Ans: C Section Ref: Special Topic 10.1
classpath
An enviroment variable that includes an argument set telling the Java virtual machine where to look for user defined classes/packages.
b
b
RuntimeException
Class is the superclass of those exceptions that can be thrown during normal runtime of the Java virtual machine.
JDK
A bundled set of development utilities for compiling, debugging, and interpreting Java Applications.
Error
This Class indicates issues that an application should not try to catch. Subclass of the class Throwable.
//
Used to comment out a line.
main loop
public static void main (String[] args)
J2EE
Acronym for Java 2 Platform, Enterprise Edition.
declare and define command line integer argument, a
int a = Integer.parseInt (args[0]);
The ____reserved word is used to deactivate polymorphism and invoke a method of the superclass. A) this B) my C) parent D) super
Ans: D Section Ref: 10.3
java.net.*;
To develop a networking application.
final
variable is defined with the reserved word its value can never change. Ex: final double .canVolume = 0.335;
java.io.*;
To utilize data streams.
JRE
An environment used to run Java Applications. Contains basic client and server JVM, core classes and supporting files.
print (return) a command line input
System.out.println (args[0]);
b
bb
byte
Used to define a primitive variable as an integer with 1 byte of storage. Also a corresponding wrapper class.
Which of the following is true regarding subclasses? A) A subclass may call the superclass constructor by using the this reserved word. B) A subclass cannot call the superclass constructor. C) If a subclass does not call the superclass constructor, it must have a constructor with parameters. D) If a subclass does not call the superclass constructor, it must have a constructor without parameters.
Ans: D Section Ref: 10.4
String
Can use +, ., and +=
Consider the following code snippet: public class Coin { . . . public boolean equals(Coin otherCoin) { . . . } . . . } What is wrong with this code? A) A class cannot override the equals() method of the Object class. B) The equals method must be declared as private. C) A class cannot change the parameters of a superclass method when overriding it. D) There is nothing wrong with this code.
Ans: C Section Ref: 10.7.2
The ____ reserved word in a method definition ensures that subclasses cannot override this method. A) abstract B) anonymous C) final D) static
Ans: C Section Ref: Special Topic 10.2
Section 16
(50 cards)
Declare and define integer a from double, b.
int a = (int) b;
not equal
!=
pen size to 0.05
StdDraw.setPenRadius(0.05);
set pen color
StdDraw.setPenColor();
sin x
Math.sin(x);
draw circle at (x, y) radius = r
StdDraw.circle(x, y, r);
cos x
Math.cos(x);
draw an image, starfield.jpg at (0, 0)
StdDraw.picture(0, 0, "starfield.jpg");
arc sin x
Math.asin(x);
square root of a
Math.sqrt (a);
to swap values of integers you need a...
place holder
declare and initial double array, points, with length N
double[] points = new double [N];
play 2001.mid
StdAudio.play("2001.mid");
draw filled circle at (x, y) radius = r
StdDraw.filledCircle(x, y, r);
declare and initial string array, names, with length N
String[] names = new String [N];
read double
StdIn.readDouble();
if / else
if (true) { } else { }
degrees to radians
Math.toRadians();
x^y
Math.pow (x, y);
draw point at (x, y)
StdDraw.point(x, y);
for loop (use i and N)
for (int i = 0; i < N; i++) { }
short hand a = a * n
a *= n;
short hand a = a + 1;
a++;
std print
StdOut.println();
and
&&
draw line from (x, y) to (q, s)
StdDraw.line(x, y, q, s);
first line of program, HelloWorld
public class HelloWorld
π
Math.PI;
declare and initial int array, number, with length N
int[] number = new int [N];
while
while (true) { }
random number between 1 and n
1 + Math.random() * n;
find the max
Math.max (x, y);
double b = a / b, but a is an int. Cast int to double
double b = (double) a / b;
arc cos x
Math.acos(x);
draw a filled rectangle centered at (x, y) width = w height = h
StdDraw.filledRectangle(x, y, w, h);
random number between 0 and 1
Math.random();
declare and define a double, a, from command line argument
double a = Double.parseDouble(args[0]);
remainder
%
read an integer
StdIn.readInt();
radians to degrees
Math.toDegrees();
short hand a = a + n;
a += n;
find the min
Math.min (x, y);
short hand a = a - 1;
a--;
set x and y window scale to -R , R
StdDraw.setXscale(-R , R); StdDraw.setYscale(-R, R);
short hand a = a - n;
a -= n;
random number between 0 and n
Math.random() * (n + 1);
absolute value of x - y
Math.abs(x - y);
draw "text" at (x, y)
StdDraw.text(x, y, "text");
boolean for empty
StdIn.isEmpty()
if statement (many commands)
if (true) { }
Section 17
(50 cards)
Modulus
Represented by the character %, gives the remainder of a division between integral or fractional numbers (note that the result can also be fractional).
'if' statement
Of the form 'if( [logical_expression] ) { statements; }'. where the statements in the body are executed if the logical expression argument in the header parentheses evaluates to true. Note that the entire structure is considered a single statement.
Primitive Variables
Can store data values.
Arithmetic Expression Type
Depends on the types of the variables or literals used in the expression. E.g 3/4 would be 0, integral, and the fractional part discarded. However if an expression mixes integrals and floating points, the expression will return a floating point type.
Java Type Categories
Primitive Types & Reference Types. It is possible for the user to create further reference types but there is a limited number of pre-defined Java primitive types.
Conditional Processing
Whereby a program can respond to changes due to the data on which it operates.
Caching truth or falsity
Rather than constantly checking the truth of a logical expression, sometimes it is better to store the result in a boolean and use that subsequently. For example 'boolean isFlying = height > 0.5f; if(isFlying == true) { flapWings(); }'
Selection Statements
Flow control structures that allow us to select which segments of code are executed depending on conditions. Includes 'if...else' and 'switch...case'.
Increment and Decrement Unary Operators
Operators available in postfix and prefix forms. The postfix version returns the old value of the variable before applying the addition or subtraction of 1. Note that these operators do change the variable that the y operate on, as well as returning values for the expression to use. E.g. 'int a = myInt++;' where a will be assigned the old value of the variable and the variable will also be incremented.
Arithmetic Operators
Can be applied to integral or floating point types, includes multiplication(*), division(/), remainder(%), addition(+), subtraction(-).
Precedence and Parentheses
Operations in parentheses are carried out first and can therefore be used by the programmer to control the order in which an expression is evaluated. This is preferable so that anyone using the code doesn't have to remember orders of precedence and it helps avoid obfuscation.
Operator Precedence
When an expression includes one or more operators, this governs in what order the operations are carried out. For example, multiplication has a higher precedence than addition and so is carried out first. Each language has rules of precedence for all of it's operators.
Casting Float Types to Integral Types
Always requires a cast. Will result in the loss of the fractional part of the number.
Promotion
Term used when casting a smaller memory type to a larger memory type and therefore a cast is not needed (in the reverse case, a cast is needed).
Initialization
When you first store something in a variables memory location.
String method length()
Returns the number of characters in a string, counting from 1. E.g. "12345".length() would return 5. An empty String has length 0.
Logical Operators
Operate on boolean values and can be unary or binary. Often used to combines logical expressions with relational operators.
Strings and escape sequences
Can be included in a string as if they were another character using the backslash \. E.g. "\"This is a string\"" uses \" to include double quotation marks as characters in the sequence.
Floating Point Literals
Either written with a whole and fractional part separated by a dot '45.98' or with scientific notation which allows significant digits that are multiplied by a power of 10 specified after the character 'e'. 'double speedOfLight = 2.998e8'. It seems that both double and float use the same literals, unlike C++.
if...else if... statement
Possible to chain many if..else statements by using 'else if...' statements. In the form 'if(logical_expression) { statements; } else if (logical_expression) { statements; } else { statements; }'. Note that you can have as many 'else if' statements as you like, and you don't have to end with an else code block.
Data Type Sizes
Fixed in Java, to ensure portability across implementations.
Statement
A unit of executable code, terminated by a semi-colon.
Operator Overloading
Where the user can define the behaviour of an operator depending on the types of its operands. This isn't possible in Java, although the '+' operator is overloaded by the system to concatenate String objects.
Prefix Operator
Unary operator that appears before it's operand. E.g. the negation operator '-' which negates the value to it's right. e.g. 'int negative speed = -speed;'
Reference Variables
Can't store data values, but refer to objects that do.
String Literal
Enclosed in double quotation marks. E.g. "Jonathan withey". This actually creates a String object to be referenced, for example 'String aString = "Character Sequence";'.
Relational Operators and Floating Points
Floating point values have limited precision, they are approximations, therefore using relational operators requires care (e.g. might be best to check within a precision range).
Code Block
Delimited by curly-brackets, enables a set of statements to be treated as a single statement.
Reference Variable
An identifier that represents a location in memory which itself has a reference to another location in memory where an object resides. Multiple reference variables can reference the same object and so affect its reference count. An object with no references can be garbage collected.
Demarcation and indentation
Good programming style in Java includes clearly showing the body of an if statement by creating a boundary around it with '{..}' curly-brackets. Segments of code can also all start with tabbed whitespace to highlight the structure of a program.
Logical Expression
Evaluates to either 'true' or 'false' (boolean literals). Can be used for conditional processing in flow control structures.
Declaration
Creating a variable of the type that you want to use, reserves a memory location for that variable.
Logical Type
boolean, with literals 'true' and 'false'. A logical variable can hold the result of a logical expression.
Strongly Typed
Whereby, in a language (Java), every variable and expression has a type when the program is compiled. You cannot arbitrarily assign the value of one type to another.
Empty String
A special string literal, denoted by "".
flow control structures
Allow different segments of code to be executed under different circumstances. Keywords : if...else, switch..case (all selection statements).
Escape Sequence
characters marked by a backslash \ that represent special information such as non-standard unicode characters, e.g. '\u00A9'. Also non-printable characters, e.g. newline '
'.
Integral Types
Primitive types, all signed : byte, short, int, long. Fixed length, regardless of platform. Can all take integer literals. An integer variable can hold the result of an arithmetic expression.
String operator '+'
Used for concatenation. If either operand is a String type, both operands will be turned into strings, concatenated and then added to a newly created String object. E.g. String aString = "Jon" + 2; would output 'Jon2'.
if...else statement
The if statement can be extended to include an alternative code block to be executed if the header argument evaluates to false. For example 'if(isFlying) { flapWings(); } else { closeWings(); }'.
Nesting Conditional Constructs
Within a code block, additional flow control structures can be nested. For example an if statement could be nested within the code block of another if statement, or a switch statement within the case of another switch statement. Note that stylistically, nesting results in further indentation so that flow can be followed easily.
Casting
The process of explicitly converting one primitive type to another. Involves writing the desired type of an expression in parentheses e.g. '(int)'. This can result in the loss of information and therefore is worth avoiding by choosing appropriate types.
Literals
Have a fixed value, as opposed to a variable. Each primitive type has an associated set of literal values and a literal value can be assigned to a primitive variable of the same type.
Data Type 'String'
Sequence of characters, a reference data type.
Operators
Sets of which are associated with primitive datatypes, can be either unary or binary, depending on whether they require one variable or two (operands). E.g. addition, multiplication or increment.
curly brackets and ;
In general, Java doesn't require code blocks delimited by '{..}' to be followed by a ';' as would normally be the case for a statement.
Local Variables
variables declared within a method or, more accurately, within a code block. These variables can only be used within the code block scope. They aren't initialised by default, so should be initialised by hand !.
Relational Operators
Check relationships between values of the same type and will be part of a logical expression. Can be used with floating points, integers and characters (including >, <, <=, >=). booleans only use '==' and '!=' which can also be used with other types.
Char Type
Store single unicode characters (international character set) and can be initialised with character literals enclosed in '. E.g. 'a'.
Floating Point Types
Those that contain fractional parts using significand digits and an exponent which represents its magnitude. Includes floats and doubles (32 and 64 bits respectively).
Section 18
(50 cards)
This method is the engine of a threaded class.
run()
internal problems involving the Java virtual machine (the runtime environment). These are rare and usually fatal to the program; there's not much that you can do about them.
Errors
What are the two subclasses of the throwable class?
Error & Exception
A thread can be created in two ways:
by subclassing the Thread class or implementing the Runnable interface in another class.
In particular, exceptions of either the ______or ____________ class or any of their subclasses do not have to be listed in your throws clause.
Error & RuntimeExceptions
Exception that occurs when a URL isn't in the right format (perhaps a user typed it incorrectly)
MalformedURLException
when writting multiple catch clauses you should always start with ______
more specific subclass exceptions and end with more general superclass exception catches (because if you start with a superclass exception catch you wont know what the specific exception was.
parts of a program set up to run on their own while the rest of the program does something else.
Threads
__________ method cannot throw more exceptions (either exceptions of different types or more general exception classes) than its superclass method.
Subclass
This method is present in all exceptions, and it displays a detailed error message describing what happened.
.getMessage() method
if a method might throw multiple exceptions you declare them all in the throws clause separated by ______
commas
!
logical NOT operator is unary and returns the opposite of its operand. Ie. if operand is true, returns false and vice-versa. E.g. 'boolean isWoman = !hasBeard;' where isWoman would equal false if hasBeard is true.
switch example
'char control = 'a'; switch (control) { case 'a': invokeA(); break; default: invokeError(); break; }' Note that because 'a' is logically equivalent to the switch argument 'control', that case would be selected and, once the statements executed, break would transfer control to the end of the switch statement, skipping the default: case.
switch....case use-age
Multi-coniditional branching. Used instead of the 'if' statement when a large number of outcomes need to be tested. Each case contains code that is executed if the switch argument takes on a particular value.
In Java, the strange events that might cause a program to fail are called
exceptions
exceptions that usually occur because of code that isn't very robust.
Runtime Exceptions
In Java, the strange events that might cause a program to fail are called
exceptions
What are the two subclasses of the throwable class?
Error & Exception
displays the sequence of method calls that led to the statement that generated the exception.
printStackTrace()
contains the class of exception to be caught and a variable name.
catch clause
How can you specify a string to make a assert statement more meaningful
assert price > 0 : "Price less than 0.";
switch default case
Can be included to perform some processing if none of the other cases in the switch statement are selected, such as throwing an exception. It isn't a requirement but is good style.
&&
logical AND operator returns true if both its operands are true. E.g. 'boolean isCup = hasHandle && holdsTea;'. Otherwise returns false.
exceptions happen when you try to use a variable that doesn't refer to an object yet.
NullPointerException
switch...case form
'switch ([argument]) { case [selector]: [statements;] break; default: [statements;] break; }'. Where many cases can be written and the 'default' case is selected if none of the other cases are selected.
if your method has been declared with a throws clause, don't forget to:
actually throw the exception in the body of your method using the throw statement.
Calling a thread's _____ method causes its _____ method to be called
start() & run()
If a class implements the Runnable interface, what methods must the class contain?
run()
^
logical XOR, effectively returns true if both its operands are different (ie. a mix of true and false) but returns false if they are the same. E.g. 'boolean isHetero = likesWomen ^ likesMen;'.
exception that is thrown when your not properly checking to make sure that your code always stays within the bounds of an array
ArrayIndexOutofBounds
What class should be the superclass of any exceptions you create in Java?
Exception
switch break
Causes the switch to terminate by transferring control to the end of the statement. Note that it can be used in a similar way in any statement.
What keyword is used to jump out of a try block and into a finally block?
return
True or False, by catching a exception superclass you can also catch any subclass exceptions that are thrown?
True
switch fall-through
If a 'break' keyword is missing after a case, processing will continue through to the next case and until it reaches a break statement or the end of the switch statement.
How does the substring() method of the String class work ?
the first argument specifies the index of the first character to include, the second argument indicates the index of the last character plus 1. A call to substring(2, 5) for a string would return the characters from index position 2 to index position 4.
switch argument
Has to be an expression of type int, char, short or byte. Notably not float, double and long (too imprecise or large).
If some code in your method's body might throw an exception, add the ______ keyword after the closing parenthesis of the method followed by the name or names of the exception
thows
What try and catch effectively mean is:What try and catch effectively mean is:
"Try this bit of code that might cause an exception.If it executes okay, go on with the program. If the code doesn't execute, catch the exception and deal with it."
adding a throws clause to your method definition simply means that the method _______ throw an exception if something goes wrong,
might
subclasses of the RuntimeException and Error classes and are usually thrown by the Java runtime itself.
Unchecked exceptions
To run a thread what method must be callled?
start() method
expressions that represent a condition that a programmer believes to be true at a specific place in a program.
assertions
Truncation
When casting a larger type to a smaller type there may be a loss of information due to the truncation of additional bits or the fractional part of the number when casting from a floating point number to an integral number.
to "catch" a exception you must surround it with a ____ & _______
try & catch
Exception that occurs when you're reading from a file and the file ends before you expected it to end.
EOFException
optional 3rd block of code in a try/catch that executes NO MATTER WHAT:
finally{ }
||
logical OR operator returns if any of its its operands are true (including both). E.g. 'boolean isCool = hasCar || hasBand'.
switch selector value
Has to be a constant value (usually a literal) that is compatible with the argument type. The statements within the case are executed if it is logically equivalent to the argument '=='.
The assert keyword must be followed by one of three things:
an expression that is true or false, a boolean variable, or a method that returns a boolean.
Section 19
(50 cards)
In particular, exceptions of either the ______or ____________ class or any of their subclasses do not have to be listed in your throws clause.
Error & RuntimeExceptions
What keyword is used to jump out of a try block and into a finally block?
return
__________ method cannot throw more exceptions (either exceptions of different types or more general exception classes) than its superclass method.
Subclass
Exception that occurs when a URL isn't in the right format (perhaps a user typed it incorrectly)
MalformedURLException
to "catch" a exception you must surround it with a ____ & _______
try & catch
when writting multiple catch clauses you should always start with ______
more specific subclass exceptions and end with more general superclass exception catches (because if you start with a superclass exception catch you wont know what the specific exception was.
Basic definition of class for Book (library program)
public class Book { // properties private String title = null; private String author = null; private boolean isCheckedOut = false; // behavior public void checkOut () { isCheckedOut = true; } public void checkIn () { isCheckedOut = false; } }
contains the class of exception to be caught and a variable name.
catch clause
expressions that represent a condition that a programmer believes to be true at a specific place in a program.
assertions
The assert keyword must be followed by one of three things:
an expression that is true or false, a boolean variable, or a method that returns a boolean.
This method is the engine of a threaded class.
run()
The Java Software Development Kit (JDK)
A set of command line tools (e.g., compilers, de-compilers, debuggers) used to develop Java programs. The JDK also contains the JRE, which contains the JVM.
Java bytecode
A platform independent instruction set that's produced when Java source code is compiled.
if a method might throw multiple exceptions you declare them all in the throws clause separated by ______
commas
Exception that occurs when you're reading from a file and the file ends before you expected it to end.
EOFException
What is the syntax of a looping control flow example?
for (int i=1; i<10; i++) { ... } while (x < y) { ... } do { ... } while (x<y)
Declare the data type of a variable.
Primitive types are used to declare the data types of variables where a variable is a named memory location. datatype variableName = initialvalue; The following statements illustrate the use of Java's eight primitive data types in variable declarations: char c = 'a'; Boolean succeeded = false; byte age = 0; short index = 0; int ssn = 0; long length = 0; float pi = 3.14159f; double d = 0.0; floating point literals are defaulted to a "double" data type. To compile certain types, such as float, add a suffix to the literal: float pi = 3.14159f;
internal problems involving the Java virtual machine (the runtime environment). These are rare and usually fatal to the program; there's not much that you can do about them.
Errors
Calling a thread's _____ method causes its _____ method to be called
start() & run()
The Java Runtime Environment (JRE)
A set of libraries that define the application programming interface (API) of the Java language. The JRE also contains the JVM application.
optional 3rd block of code in a try/catch that executes NO MATTER WHAT:
finally{ }
Are primitive data types considered objects in Java?
No, primitive types are the only elements of the Java language that are not modeled as objects.
What is the syntax of a decision control flow example?
if (x == y) { ... } else { ... } switch (index) { case 0: {...} case 1: {...} default: {...} }
If a class implements the Runnable interface, what methods must the class contain?
run()
What try and catch effectively mean is:What try and catch effectively mean is:
"Try this bit of code that might cause an exception.If it executes okay, go on with the program. If the code doesn't execute, catch the exception and deal with it."
JSE JEE JME Java Card
Java Standard Edition - Contains the core functionality of the Java language; used to develop desktop applications. Java Enterprise Edition - Provides additional functionality required of enterprise applications, including web applications. Java Micro Edition - A scaled-down version used for mobile phones and other handheld devices. Java Card - The smallest Java footprint used for integrated circuits (e.g., memory and microprocessor cards).
If some code in your method's body might throw an exception, add the ______ keyword after the closing parenthesis of the method followed by the name or names of the exception
thows
True or False, by catching a exception superclass you can also catch any subclass exceptions that are thrown?
True
How can you specify a string to make a assert statement more meaningful
assert price > 0 : "Price less than 0.";
Syntax to define a method (behavior (function in C++))
returntype methodName(optional_list_of_arguments) { ...} Names of properties (variables) and methods should start with a lower case letter, using camelCase if multiple words.
UML Diagrams - list the structure diagrams
Structure Diagrams - CCCODPP Composite Structure Class Component Object Deployment Profile Package
What class should be the superclass of any exceptions you create in Java?
Exception
parts of a program set up to run on their own while the rest of the program does something else.
Threads
8 primitive data types
All of Java's numeric primitive data types are signed. Boolean (1 bit): true, false char (2 bytes): Unicode characters, ranging from 0 TO 65,535 byte (1 byte): -128 to 127 short (2 bytes): -32,768 TO 32,767 int (4 bytes): -2,147,483,648 TO 2,147,483,647 long (8 bytes): -9,223,372,036,854,775,808 TO +9,223,372,036,854,775,807 float (4 bytes): 1.40129846432481707e-45 TO 3.40282346638528860e+38 (+ or -) double (8 bytes): 4.94065645841246544e-324d TO 1.79769313486231570e+308d (+ or -)
What is the syntax of a sequence control flow example?
base = rate * hours; taxes = base * taxRate; net = base - taxes; count++; count--;
displays the sequence of method calls that led to the statement that generated the exception.
printStackTrace()
adding a throws clause to your method definition simply means that the method _______ throw an exception if something goes wrong,
might
Definition of class
Describes the properties and behavior of a prototypical object. During runtime, the application creates individual instances of classes where the instances are called objects. For example: class - Book properties - title, author behaviors - checkOut(), checkIn() Then at runtime, the application creates a separate object for each book as they are requested by users.
This method is present in all exceptions, and it displays a detailed error message describing what happened.
.getMessage() method
Syntax to define a class
public class SomeClassName { // class properties and behaviors (methods) go here } keyword "public" is an access modifier, keyword "class" identifies the construct (i.e., the item) as a class definition, class name may contain alpha numeric characters, underscore, and $ (last two are rarely used). By convention, class names always start with an upper case letter, using CamelCase if there are more than one word.
exception that is thrown when your not properly checking to make sure that your code always stays within the bounds of an array
ArrayIndexOutofBounds
Java Virtual Machine (JVM)
A platform-targeted runtime environment that knows how to execute Java bytecode.
To run a thread what method must be callled?
start() method
if your method has been declared with a throws clause, don't forget to:
actually throw the exception in the body of your method using the throw statement.
exceptions that usually occur because of code that isn't very robust.
Runtime Exceptions
How do you declare a CONSTANT in Java?
Using the keyword "final": final float pi = 3.14159f;
A thread can be created in two ways:
by subclassing the Thread class or implementing the Runnable interface in another class.
subclasses of the RuntimeException and Error classes and are usually thrown by the Java runtime itself.
Unchecked exceptions
How does the substring() method of the String class work ?
the first argument specifies the index of the first character to include, the second argument indicates the index of the last character plus 1. A call to substring(2, 5) for a string would return the characters from index position 2 to index position 4.
exceptions happen when you try to use a variable that doesn't refer to an object yet.
NullPointerException
Section 20
(50 cards)
Application defined classes
classes that you create for your specific application Login -username : String -password : String +setUsername : (username : String) +getUsername () : String +setPassword : (password : String) +getPassword () : String +validate() : boolean +equals(Login Login) : boolean two private data members: username and password; the minus sign means the properties are hidden in the class, cannot access them outside of class. When a class does not explicitly provide a constructor, Java implicitly provides a default constructor (one that takes no arguments). The default constructor is available for use even though it's not shown.
JSE JEE JME Java Card
Java Standard Edition - Contains the core functionality of the Java language; used to develop desktop applications. Java Enterprise Edition - Provides additional functionality required of enterprise applications, including web applications. Java Micro Edition - A scaled-down version used for mobile phones and other handheld devices. Java Card - The smallest Java footprint used for integrated circuits (e.g., memory and microprocessor cards).
Queue Interface
represents an ordered collection based on some known order: e.g., first in first out (FIFO), last in first out (LIFO, a.k.a. a stack), priority. A subset of the Queue API is shown below (not including the methods inherited from interface Collection). <<interface>> Queue +element() : E +offer(E) : boolean +peek() : E +poll() : E +remove() : E order of the queue is determined by the implementing class. Classes that implement the Queue interface include LinkedList and PriorityQueue. As an example, suppose the Library system solicits user suggestions, and these suggestions need to be processed in a FIFO basis. private Queue suggestions = new LinkedList(); public void addSuggestion(Suggestion suggestion) { suggestions.add(suggestion); } The above method adds the new Suggestion to the Queue. Because the implementing class is a LinkedList, each suggestion is added to the end of the queue.
Keyword: new
new - creates Class objects and a constructor as shown below: Book book1 = new Book(); Book book2 = new Book("SomeTitle", "SomeAuthor", false); Constructors are invoked with the "new" keyword as shown below: Book book1 = new Book(); Book book2 = new Book(1); Book book3 = new Book(1, "Gone with the Wind", "Margaret Mitchell"); Using a constructor that allows you to initialize the object with passed-in parameters. Using the default constructor, followed by the invocation of set(...) methods.
Keyword: this
this - used to disambiguate the overloaded use of variable names; in particular, "this" can be used to reference a data member (defined in the class) when that name is identical to a passed parameter. assigns the input parameter, title, to the data member, title: private String title = ""; public void setTitle(String title) { this.title = title; } inside the method setTitle(String title), the passed parameter, title, hides the data member, title, defined in the class. To access to the title data member, the keyword "this" is used to indicate a data member of the object is being referenced (and not a local variable inside the method).
Defining Exceptions
exceptions are classes that extend the class Exception either directly or indirectly. For example, just a few of the many exceptions defined by the Java API. public class ClassNotFoundException extends Exception {...} public class IOException extends Exception {...} public class NullPointerException extends Exception {...} public class NumberFormatException extends Exception {...} The Exception class, most of its behavior is inherited from a base class named Throwable, which defines a message (String) property and a corresponding accessor, getMessage(). defining a custom exception that denotes a failed login: public class LoginFailedException extends Exception { public LoginFailedException() { super(); } public LoginFailedException(String msg) { super(msg); } } when defining your own exceptions, provide at least two constructors, default constructor and a constructor that takes a message string as a parameter. In both cases, the base constructor of Exception is invoked using the keyword super.
List Interface
represents an ordered sequence of objects. That is, elements are added to the list in a particular order, and the list maintains that order. suppose we have a List, booksCheckedIn, which contains all the books that are checked-in on a given day. Then, if we add books to the list as they are checked-in, the list will maintain the order in which they were checked-in. private List booksCheckedIn = new ArrayList(); public void checkin(Book book) { booksCheckedIn.add(book); } Implementations of the List interface include ArrayList, LinkedList, Stack, and Vector. List collections do not check for duplicate entries, so if an object is added multiple times, it will appear in the list multiple times (i.e., have multiple entries). Therefore, if it's important for a given object to appear only once in the collection, the Set interface should be used as explained below.
Java Collection Framework
Group of interfaces, classes, and algorithms that together provide a rich set of abstractions for grouping objects. Maps - A collection whose entries are accessed by some unique identifier. Lists - A collection whose entries can be accessed with an index. Sets - A collection whose entries are guaranteed to be unique. Queues - A collection whose entries are ordered, based on some scheme.
Objects example illustrated
CLASS Book title: String author: String isCheckedOut: boolean checkOut(): void checkIn(): void If a user checks out 2 books, each of them are assigned the class properties: Book Book title: Call of the Wild title: Gone with the Wind author: J. London author: M. Mitchell isCheckedOut: true isCheckedOut: true class is a type, and objects are instances of those types
polymorphism
Method overloading is an example of polymorphism. polymorphism can be defined as "one interface with multiple behaviors." Another form of polymorphism is method overriding, which is an example of dynamic binding. Method overriding is discussed later in this topic.
Java Virtual Machine (JVM)
A platform-targeted runtime environment that knows how to execute Java bytecode.
Package
Items of related functionality, each item in the package has a unique name. Thus, a package is a namespace, where a namespace is a collection of uniquely named items. One of main packages is "java", does not contain any data types, it's used to aggregate other packages that are nested within it. java.applet java.awt java.beans java.io java.util nested package example: java.util.concurrent.locks other nested packages do contain type definitions: - java.util contains the Date class (and other classes) - java.util.concurrent contains the Semaphore class (and other classes) - java.util.concurrent.locks contains the LockSupport (and other classes)
The Java Software Development Kit (JDK)
A set of command line tools (e.g., compilers, de-compilers, debuggers) used to develop Java programs. The JDK also contains the JRE, which contains the JVM.
Class Constructors
special methods that are called when a class is created. Must have the same name as the enclosing class, and are declared with no return value (the implied return type of the constructor is the enclosing class type). public Book() {...} public Book(int id) {...} public Book(int id, String title) {...} public Book(int id, String title, String author) {...} public Book(int id, String title, String author, int pages) {...} Constructors can be overloaded. Once you define at least one constructor (whether default or not), the implicit, hidden default constructor is not provided for you. used to initialize the data members of the newly created object either by assigning default values or by using the passed-in parameters. Constructors can also invoke other constructors either in the same class or in a base class. public Book(int id) { super(id); } Above uses "super" keyword to invoke a constructor in the base class LibraryItem. Constructors are invoked with the "new" keyword
Importing packages
when classes are in defined in different packages. In order for ClassA, in packageA, to have visibility to ClassB, in packageB, one of three things must happen; either: - the fully qualified name of ClassB must be used, - fully qualified name of ClassB be must be imported with an import statement, - entire contents of packageB must be imported with an import statement. first technique is to use the FQN of the class. The FQN of ClassB is packageb.ClassB. to declare: package packagea; public class ClassA { packageb.ClassB b = new packageb.ClassB(); } better technique is to use an "import" statement: package packagea; import packageb.ClassB; public class A { ClassB b = new ClassB(); } import statements must appear after the package statement, and before the data type (class, interface) definition more efficient technique is to import all the data types from a given package using the * notation: import packageb.*; if multiple packages have classes with the same name, use long way (FQN) of importing package: package1.ClassXYZ xyz = new package1.ClassXYZ()
Method overloading
occurs when a class contains more than one method with the same name. Within a class, methods can have the same name if there's difference: The number of parameters. And/or the type parameters. And/or order of parameters. The determination of which overloaded method to invoke is a compile-time decision; hence, it is sometimes referred to as static binding. Library library = new Library(); Book book = new Book(); library.add(book); User user = new User(); library.add(user); Notice that the statement library.add (book) is clearly targeting the add (Book) method because of the parameter data type, Book. Similarly, the statement library.add(user) is clearly targeting the add(User) method because of the parameter data type, User. Since these decisions can be determined at compile time, the binding is said to be static.
Set Interface
represents a collection of objects with no duplicates. By default, duplicates are identified by their equals(Object) method whose default behavior is defined in the root class Object. In other words, if equals(Object) is not overridden, object references will be used to determine if list entries are equal. If this behavior is not desired, equals(Object) needs to be overridden in the class for which objects are added to the set. A portion of the Set API is shown below. <<interface>> set +add(E) : boolean +addAll(Collection) : boolean +clear() : void +contains(Object) : boolean +containAll(Collection) : boolean +isEmpty() : boolean +remove(Object) : boolean +size() : int suppose the Library system has a business rule that prevents users from checking out more than one copy of a given book. During the checkout process, the user's books could be added to a Set collection where duplicate books are not added to the set. private Set booksCheckedOut = new HashSet(); public boolean checkout(Book book) { return booksCheckedOut.add(book); } The above method returns true if the book is not already in the set; otherwise, it returns false. If false is returned, the user would be notified that the duplicate book could not be checked out. Implementations of the Set interface include HashSet and TreeSet. The order of elements in a Set depends on the underlying implementation, and thus, will vary across implementations. If order is important, the interface SortedList should be used.
The comparing method: equals(Object)
Using equals(Object): boolean areEqual = book1.equals(book2); It compares object references, compares to see if book1 and book2 reference the same Book object in memory. In the example above, book1 and book2 reference different Book objects, so the boolean variable areEqual is set to false. This one would be true: Book book1 = new Book(); Book book2 = book1; boolean areEqual = book1.equals(book2);
Associations
allow classes (and their objects) to form relationships with one another. These relationships facilitate object collaboration. two types of associations: "Has a" relationships: Abstracts/models the concept of containment (e.g., a Customer has an Account; or in other words, the Customer contains an Account). "Uses a" relationships: Abstracts/models the concept of using (e.g., a Workflow manager uses various services to accomplish a task [to get some work done]). "has a" relationship with another class if it has a data member of that particular type. package domain; public class User { private Account account = new Account(); } abstracting a Login class that contains the user's credentials (username, password) and associating it with the User by declaring Login data member: package domain; public class User { private Login login = new Login(); private Account account = new Account(); } Once you declare a data member as an object type, you have essentially created a "has a" relationship between the two objects. "uses a" relationship, only difference between a "has a" and "uses a" relationship is where the declaration is made. "uses a" relationships are defined within method declarations (and not as data members of the enclosing class). So for instance, suppose a particular method needs to use a Date object to timestamp a message: package domain; public class Logger { public generateLogMessage(String message) { Date date = new Date(); } } The above method instantiates a Date object locally within the scope of the method, not as a data member (at the class/object level). "uses a" relationship. So we can summarize "has a" and "uses a" relationships as follows: "Has a" relationships are realized as object declarations located at the class level (i.e., as data members). "Uses a" relationships are realized as object declarations located within the scope of a method.
Arrays
Group (or collection) of items that are the same type. declares an array object, daysOfWeek, as an array of seven String objects: String [ ] daysOfWeek = new String[7]; assign the seven elements as follows: daysOfWeek[0] = "Sunday"; daysOfWeek[1] = "Monday"; daysOfWeek[2] = "Tuesday"; daysOfWeek[3] = "Wednesday"; daysOfWeek[4] = "Thursday"; daysOfWeek[5] = "Friday"; daysOfWeek[6] = "Saturday"; values could be initialized at the point of declaration: private String [ ] daysOfWeek = {"Sunday", "Monday", "Tuesday", "Wednesday" , "Thursday", "Friday", "Saturday"}; The size of the array is available via the length property (e.g., daysOfWeek.length), allowing you to iterate through its values in a "for loop": for (int i = 0; i<daysOfWeek.length; i++) { System.out.println(daysOfWeek[i]); } Array downside, once declared, their size is fixed. Can create a new, larger array and move the contents of the first array into the second, but that is clumsy. Typically used only for grouping items whose size is fixed.
Declaring a class to a package
class is declared to belong to a package by including a package statement at the beginning of the source file, it must be the first statement in the file: package mypackage; public class MyClass { ... } If package is not declared, the class will belong to the "default package". Not recommended for larger programs.
Constructor syntax
public class Book { // properties private String title = null; private String author = null; private boolean isCheckedout = false; // constructors public Book() { } public Book(String title, String author, boolean isCheckedout) { this.title = title; this.author = author; this.isCheckedout = isCheckedout; } } Two constructors: Book(), which is known as the default constructor Book(String title, String author, boolean isCheckedout) Default constructor - typically used to initialize properties to their "null" values, unless it is done when they are declared, then there is no need to have the first constructor. The 2nd constructor above, takes the passed parameters and assigns them to the class's properties (i.e., data members) using the keyword "this".
Access Modifiers
Information hiding is achieved through the use of access modifiers, which are keywords that are used to augment the visibility of various constructs. Public Default (no modifier) Protected Private
Throwing & Catching Exceptions
Exceptions can be thrown either by the Java API or by your code. To throw an exception examples: throw new Exception(); throw new Exception("Some message goes here"); throw new LoginFailedException("User not authenticated"); Once an exception is thrown, the JVM starts looking for a handler in the call stack to transfer execution of the program. Exception handlers are declared in the context of a try/catch block. Catching Exceptions Java supports the catching of exceptions in what's known as a try/catch block or also known as a try/catch/finally block. Syntax for a try/catch block: try { ... } catch (SomeException e) { ... } The interpretation of the try/catch block is the following: the statements between the try and catch are executed sequentially, and if no exception occurs, then execution continues with the first statement that follows the try/catch block. On the other hand, if an exception occurs while executing the statements between the try/catch, execution transfers to the catch block, and then once the catch block finishes executing, execution continues with the first statement after the try/catch block.
Inheritance
One of the 3 main pinnacles of object-oriented programming (the others being encapsulation and polymorphism). Inheritance is a means by which a class extends another class, it acquires the properties and behavior of the class that's being extended. Sometimes called generalization/specialization, also called an "is a" relationship. For example, the class declarations of Book, Audio, and Periodical are shown below: package domain; public class Book extends LibraryItem { // properties and behavior of Book go here } package domain; public class Audio extends LibraryItem { // properties and behavior of Audio go here } package domain; public class Periodical extends LibraryItem { // properties and behavior of Periodical go here } class can only extend one other class -- cannot extend multiple classes
Objects...
are instances (instantiations) of classes, as objects are created, the properties that are defined in the class template are copied to each object instance and assigned values. Thus, each object has its own copy of the class properties
Declaring Exceptions
required that if a method throws an exception without catching it, the exception must be declared in the method signature to notify users. method that authenticates logins, throws an exception named LoginFailedException without catching it: public bool authenticate(Login login) throws LoginFailedException {...} Any exception that occurs in a method must either be handled (caught) by that method or be declared in its signature as "throws" (see above). method throws (and does not handle) multiple exception types, instead of listing all the exceptions in the method signature, the base exception class Exception can be listed: public bool authenticate(Login login) throws Exception {...} declaration covers all possible exception types, and thus prevents you from having to list the individual exceptions when multiple exceptions can be thrown.
Iterable Interface
base interface for all interfaces of the Java collection framework (excluding Map). Iterable defines exactly one method, namely: Iterator iterator(); This allows you to get an Iterator for iterating over the elements of a collection. Since all other collection interfaces extend Iterable (either directly or indirectly), they all inherit the above method; that is, they all provide an iterator for iterating over their elements. For example, the Collection interface directly extends Iterable, and hence, if you have a Collection, you can get an iterator to iterate over its elements: Collection<Book> coll = getOverdueBooks(); Iterator<Book> iter = coll.iterator(); while (iter.hasNext()) { Book book = iter.next(); } Because the need to iterate over all items in a collection is a common practice, the method iterator() is placed in the base interface Iterable, and then through inheritance, it's made available to all collections that extend Collection (because Collection extends Iterable). The extension of Iterable by Collection (and then List, Set, and Queue) is known as interface inheritance.
Predefined Java classes
Example of 2: Object class & String class
Method Overriding
when a method in a base class is repeated (redefined) in a derived class signature of both methods must match identically override its behavior in the LibraryItem to output the item's id and title as shown below: public class LibraryItem { ... public String toString() { return "LibraryItem, id=" + id + ", title: " + title; } } can invoke toString() to get a meaningful description of the object. Also true if the object is a Book, Audio, or Periodical because they inherit LibraryItem. Object obj = new Book(1, "Catch-22", "Joseph Heller", 485); String s = obj.toString(); actual instance is a Book and Book "is a" LibraryItem which overrides toString(), the LibraryItem's toString() method is called. Example of polymorphic dynamic binding; dynamic because it's not known until runtime which toString() method is invoked; it depends on whether the method is overridden, and if so, where. Can override toString in each of the more specialized classes: Book, Audio, and Periodical: public class Book extends LibraryItem { public String toString() { return super.toString() + ", Author, =" + author; } } super.toString() in the above return statement; its purpose is to invoke the toString() method in the base class LibraryItem. So the Book's overridden toString() method calls the LibraryItem's toString() method and then appends to it the author information that's contained in the Book. This time, since Book has overridden toString(), which overrides LibraryItem.toString(), which overrides Object.toString(), it's Book.toString() that gets called, and not the ones declared in Object or LibraryItem: Object obj = new Book(1, "Catch-22", "Joseph Heller"); String s = obj.toString(); another example of method overriding, consider the method equals(Object obj) defined by the Object class. to compare the state of two objects, overriding equals(Object obj) as shown below for LibraryItem: public class LibraryItem { public boolean equals(Object obj) { if (this == obj) return true; if ( ! (obj instanceof LibraryItem)) return false; LibraryItem item = (LibraryItem)obj; if ( this.id != item.id ) return false; if ( ! this.title.equals(item.title)) return false; return true; } } @override annotation - applied to methods that are intended to override behavior defined in a base class. annotation forces the compiler to check that the annotated method truly overrides a base class method. package domain; public class LibraryItem { @Override public String equals(Object obj) { ... } @Override public String toString() { ... } }
String class
Java's String class is used to represent a sequence (i.e., string) of characters. A simplified class diagram of String is shown below: STRING count : int value : char[] String() String(orig:String) length : int charAt(index:int) : char concat(st : String) : String equals(obj : String) : boolean indexOf(str : String) : int isEmpty() : boolean lastIndexOf(str : String) : int length() : int replace(oldChar : char , newChar : char) : String substring(beginIndex : int, endIndex : int) : String toLowerCase() : String toUpperCase() : String toString : String * Extends class Object and thus inherits Object's properties and behavior, including method equals(Object); * Encapsulates private fields (e.g., count and value), prevents direct access to these fields by other objects; indirect access is provided through public methods; e.g., the method length() provides access to the count field; * Has two overloaded constructors, String() and String(String s). The first constructor takes no parameters, and the second takes a String object that is copied into the new String object; * Overrides methods toString() and equals() inherited from class Object by providing its own implementations; * Exhibits polymorphism (one interface, multiple behaviors) with methods toString() and equals();
static fields and methods
By default, data members and methods are "instance" members and are said to be "non static." Non static instance data members are copied to each object instance of the class, so each object has its own copy. Non static methods can reference these copied data members through an implicit object reference (but you can access it via "this" when necessary). The concept of non static instance members is illustrated below. public class Book { private String title = ""; public void setTitle(String title) { this.title = title; } } When there's a need to not copy data members to each object instance, such as constants. They can remain in the class definition. Data members and methods that are not associated with object instances are call "class" members; they are local to the class and are not affiliated with the objects. Annotated with the keyword "static": public static final int CHECKOUT_DURATION = 21; Because the above data member is a class variable, it can be accessed through the class name as follows: int duration = Book.CHECKOUT_DURATION; Methods can also be designated as static: private static int nextId = 1; public static int getNextId() { return nextId++; } Above, method getNextId() provides access to the static data member and manages its next value. IMPORTANT: "static only sees static" but "non-static sees everything" static members can only access other static members; in particular, static members cannot access non-static members. Non-static members have access to all members (both static and non-static).
method toString()
default behavior is to return the object's fully qualified name followed by the object's hash code. book1.toString(); //returns: Book@1ea2df3 book2.toString(); //returns: Book@1ea2df3 this information is typically not useful, it's common to override toString().
Syntax to create a String
String s1 = new String("Hello World"); Can be created and initialized without using new: String s2 = "Hello World"; Can be concatenated with the + operator: String s3 = "Hello " + "World"; Operator == compares object references (and not the object's state), and since s1, s2 and s3 reference different String objects, == returns false: (s1 == s2); // returns false (s2 == s3); // returns false (s1 == s3); // returns false String's implementation of equals(Object) examines the values of the strings, this is true: s1.equals(s2); s2.equals(s3); s1.equals(s3); Strings objects created with the "new" operator that have the same value (the same string of characters) are always "equals()" to each other, but since they are different objects, they are never "==" to each other. One last subtle point: strings created using the double quote syntax with the same value without the new operator are always = = to each other. String s4 = "Hello World"; String s5 = "Hello World"; (s4 == s5); // returns true
The Java Runtime Environment (JRE)
A set of libraries that define the application programming interface (API) of the Java language. The JRE also contains the JVM application.
Default (no modifier)
Can be applied to a class and/or individual data members and methods within a class; indicates that the given item is not accessible across package boundaries (i.e., it limits access to members of the same package) e.g., the following class and method are NOT accessible outside the package in which they're defined. package domain; class Book { void checkout() { } }
Constructors are special methods defined in a class that are used to initialize ___ ___.
Answer: object instances Constructors must have the same name as the class name; the class Book will have constructors named Book (must be identical). Constructs are not allowed to explicitly declare that they return a type; rather, the implicit type they return is the enclosing class type in which they're defined.
Object class
Object is base class of all Java classes. All Java classes extend (inherit) class Object either directly or indirectly. these 2 definitions are exactly the same: public class Book {...} public class Book extends Object {...}
Java bytecode
A platform independent instruction set that's produced when Java source code is compiled.
Collection Interface
general abstraction for a collection of objects. It provides a handful of methods that operate over all interfaces that extend Collection. <<interface>> Collection +add(E) : boolean +addAll(Collection) : boolean +clear() : void +contains(Object) : boolean +containAll(Collection) : boolean +isEmpty() : boolean +remove(Object) : boolean +size() : int Java API does not provide any direct class implementations of interface Collection. Instead, implementations are provided for interfaces that extend Collection, namely, List, Set, and Queue. Nevertheless, the Collection interface is often used to pass collections around since it represents the base behavior of all collections. We'll now take a look at List, Set, and Queue and see how they might be used in our Library application.
Private
Can be applied to individual data members and methods (but cannot be applied to a class); when applied, it restricts access to members of the same class; e.g., the following data member, isbn, is only accessible to members of the Book class, and nowhere else. package domain; public class Book { private String isbn; public void getIsbn() { return isbn } }
Public modifier
Can be applied to a class and/or individual data members and methods within a class; indicates that the given item is accessible across package boundaries, the following class and method are both accessible outside the package in which they're defined. package domain; public class Book { public void checkOut() { } }
Maps
supports the ability to add and retrieve items from a collection using a key value pair. The key (K) is the lookup identifier and the value (V) is the item being looked up. The methods for inserting/retrieving items in/out of the Map are put(...) and get(...). General accounts - For general public use. Business accounts - For commercial organizations. Nonprofit accounts - For nonprofit organizations. The above account types can be abstracted as a Java enum (enumerator): enum AccountType {general, business, nonprofit}; Each user is allowed to have one account of each type. Accounts for a given user could be contained in a Map data structure where the key for each account is one of the above enum values (general, business, or nonprofit) as shown below: Account generalAccount = new Account(); Account businessAccount = new Account(); Account nonprofitAccount = new Account(); //... Map<AccountType, Account> accountsMap = new HashMap<AccountType, Account>(); accountMap.put(AccountType.general, generalAccount); accountMap.put(AccountType.business , businessAccount); accountMap.put(AccountType.nonprofit , nonprofitAccount); Retrieval of the accounts from the map is then achieved with the key: Account generalAccount = accountMap.get(AccountType.general); Account businessAccount = accountMap.get(AccountType.business); Account nonprofitAccount = accountMap.get(AccountType.nonprofit); In summary, the key is used to place values in the map and also to retrieve values from the same map. advantage, code readily accommodates additional account types. simply add to the enum type: enum AccountType {general, business, nonprofit, government, education};
Inner & Anonymous Classes
Inner classes are defined within the context of another class. package domain; public class Book extends LibraryItem implements AuthorableItem, PrintedMaterial { BookReader reader = new BookReader(); // .rest of Book definition public String nextLine() { return reader.readNextLine(); } // Note that this is not a "public" class- //only an instance of Book can access the BookReader class BookReader { public BookReader() { } public String readNextLine() { // Implementation of this method. } } // BookReader } // Book Anonymous classes are also inner classes that are defined within the context of another class, but these classes do not have a name, hence the name anonymous. Anonymous inner classes are often used as event handlers in the implementation of graphical user interfaces.
UML Class diagram w/ general syntax
ClassName Properties Here Behaviors Here Book title: String author: String isCheckedOut: boolean checkOut(): void checkIn(): void Each property name is followed by its data type (e.g., String, boolean). Each method name is followed by its return type (e.g., void).
Exception Handling
events (e.g., errors) that disrupt the normal flow of execution. Examples of software exceptions: - Dividing by zero. - Using an unassigned object reference (that points to null) to invoke a method. - Failure to open/read/write/close a file. - Failure to open/read/write/close a socket connection. - Database errors. when a program executes, described as a call stack, ordered list of the active methods called up to the current point in time. stack starts with the main entry point and documents the active methods that are currently under call. once an exception occurs, it must be handled, or the program aborts. the JVM marches through the call stack looking for the nearest handler for the exception. If one is found, control is transferred. Otherwise, the program terminates.
Protected modifier
Can be applied to individual data members and methods (but cannot be applied to a class); when applied, it restricts access to members of the same class and any derived class; e.g., the following method, setHashCode, is only accessible to members of the LibraryItem class and any derived classes (e.g., Book). package domain; public class LibraryItem { protected void setHashCode() { } } package domain; public class Book extends LibraryItem { public LibraryItem () { setHashCode(); } }
Abstract Classes & Interfaces
abstract class - class that is never intended to be instantiated as an object. Its purpose is to serve as a base class that provides a set of methods that together form an API. classes that inherit the abstract class can override its base behavior as necessary. Because the derived classes can be instantiated, they are referred to as concrete classes. This combination of an abstract base class and one or more concrete derived classes is widely used in industry and is known as the "Template" design pattern. class is identified as being abstract by pre-pending the keyword "abstract" to the class definition. package domain; public abstract class LibraryItem { } abstract keyword prohibits a LibraryItem from being instantiated with the "new" keyword. When a class is denoted as abstract, it typically means that one or more of its methods are also abstract, which implies they have no implementation. method is declared to be abstract by using the "abstract" keyword: public abstract boolean validate(); there's no {...} following the signature; rather, a semicolon is present. method must not have an implementation following its signature, the enclosing class must be declared to be abstract. implementation of validate() in one of the concrete derived classes: Book. package domain; public class Book extends LibrarayItem { public boolean validate() { if (id <= 0) return false; if (title == null || title.equals("")) return false; if (author == null || author.equals("")) return false; if (pages <= 0) return false; return true; } } another technique for declaring "abstract" interfaces by way of the "interface" keyword: package domain; public interface IValidator { public boolean validate(); } interface construct is used to capture zero or more method signatures without specifying their implementation details. methods enclosed within an interface definition must be "abstract" in the sense that they must not provide an implementation (i.e., {...}), but the abstract keyword is not necessary. interfaces are implemented in classes with the "implements" keyword. User and Login are not defined as abstract since implementations are provided for the validate() method. package domain; public abstract class LibraryItem implements IValidator { public abstract boolean validate(); } uniform interface for validating objects. package domain; public class User implements IValidator { public boolean validate() { } } package domain; public class Login implements IValidator { public boolean validate() { } } while Java does not support multiple class inheritance , it does support the implementation of multiple interfaces in a single class. To serialize Login objects across a secure network Login class needs to implement the Serializable interface. package domain; public class Login implements IValidator, Serializable {...} Serializable does not contain any methods, known as a Marker Interface. Marker interfaces, used to "mark" classes (and hence their objects) as "one of those," but without any additional methods. In summary, interface constructs are used to separate interface abstractions from implementation details.
UML Diagrams - list the behavior diagrams
Behavior Diagrams - STIACUS Sequence Timing Interactive Activity Communication Use Case State
Software Layers and responsibilities
* Presentation Layer - rendering the user interface, responding to user event, creating and populating domain objects, and invoking the business layer. * The Business Layer - managing workflow by coordinating the execution of services located in the service layer. * The Service Layer - service interfaces and implementations, the movement of objects in and out of the application, and the hiding of technology choices. * The Domain Layer - abstracting domain objects (i.e., the nouns) as they appear in the problem space. Data types for: user interface ->> Presentation Layer; manage use case workflow ->>Business Layer; move objects in and out of application ->>Service Layer; abstract the "nouns" of problem space ->> Domain Layer
Section 21
(50 cards)
Arrays
Group (or collection) of items that are the same type. declares an array object, daysOfWeek, as an array of seven String objects: String [ ] daysOfWeek = new String[7]; assign the seven elements as follows: daysOfWeek[0] = "Sunday"; daysOfWeek[1] = "Monday"; daysOfWeek[2] = "Tuesday"; daysOfWeek[3] = "Wednesday"; daysOfWeek[4] = "Thursday"; daysOfWeek[5] = "Friday"; daysOfWeek[6] = "Saturday"; values could be initialized at the point of declaration: private String [ ] daysOfWeek = {"Sunday", "Monday", "Tuesday", "Wednesday" , "Thursday", "Friday", "Saturday"}; The size of the array is available via the length property (e.g., daysOfWeek.length), allowing you to iterate through its values in a "for loop": for (int i = 0; i<daysOfWeek.length; i++) { System.out.println(daysOfWeek[i]); } Array downside, once declared, their size is fixed. Can create a new, larger array and move the contents of the first array into the second, but that is clumsy. Typically used only for grouping items whose size is fixed.
Syntax to define a class
public class SomeClassName { // class properties and behaviors (methods) go here } keyword "public" is an access modifier, keyword "class" identifies the construct (i.e., the item) as a class definition, class name may contain alpha numeric characters, underscore, and $ (last two are rarely used). By convention, class names always start with an upper case letter, using CamelCase if there are more than one word.
UML Class diagram w/ general syntax
ClassName Properties Here Behaviors Here Book title: String author: String isCheckedOut: boolean checkOut(): void checkIn(): void Each property name is followed by its data type (e.g., String, boolean). Each method name is followed by its return type (e.g., void).
Predefined Java classes
Example of 2: Object class & String class
Declaring a class to a package
class is declared to belong to a package by including a package statement at the beginning of the source file, it must be the first statement in the file: package mypackage; public class MyClass { ... } If package is not declared, the class will belong to the "default package". Not recommended for larger programs.
Private
Can be applied to individual data members and methods (but cannot be applied to a class); when applied, it restricts access to members of the same class; e.g., the following data member, isbn, is only accessible to members of the Book class, and nowhere else. package domain; public class Book { private String isbn; public void getIsbn() { return isbn } }
UML Diagrams - list the structure diagrams
Structure Diagrams - CCCODPP Composite Structure Class Component Object Deployment Profile Package
8 primitive data types
All of Java's numeric primitive data types are signed. Boolean (1 bit): true, false char (2 bytes): Unicode characters, ranging from 0 TO 65,535 byte (1 byte): -128 to 127 short (2 bytes): -32,768 TO 32,767 int (4 bytes): -2,147,483,648 TO 2,147,483,647 long (8 bytes): -9,223,372,036,854,775,808 TO +9,223,372,036,854,775,807 float (4 bytes): 1.40129846432481707e-45 TO 3.40282346638528860e+38 (+ or -) double (8 bytes): 4.94065645841246544e-324d TO 1.79769313486231570e+308d (+ or -)
Basic definition of class for Book (library program)
public class Book { // properties private String title = null; private String author = null; private boolean isCheckedOut = false; // behavior public void checkOut () { isCheckedOut = true; } public void checkIn () { isCheckedOut = false; } }
What is the syntax of a decision control flow example?
if (x == y) { ... } else { ... } switch (index) { case 0: {...} case 1: {...} default: {...} }
Object class
Object is base class of all Java classes. All Java classes extend (inherit) class Object either directly or indirectly. these 2 definitions are exactly the same: public class Book {...} public class Book extends Object {...}
Public modifier
Can be applied to a class and/or individual data members and methods within a class; indicates that the given item is accessible across package boundaries, the following class and method are both accessible outside the package in which they're defined. package domain; public class Book { public void checkOut() { } }
Are primitive data types considered objects in Java?
No, primitive types are the only elements of the Java language that are not modeled as objects.
Abstract Classes & Interfaces
abstract class - class that is never intended to be instantiated as an object. Its purpose is to serve as a base class that provides a set of methods that together form an API. classes that inherit the abstract class can override its base behavior as necessary. Because the derived classes can be instantiated, they are referred to as concrete classes. This combination of an abstract base class and one or more concrete derived classes is widely used in industry and is known as the "Template" design pattern. class is identified as being abstract by pre-pending the keyword "abstract" to the class definition. package domain; public abstract class LibraryItem { } abstract keyword prohibits a LibraryItem from being instantiated with the "new" keyword. When a class is denoted as abstract, it typically means that one or more of its methods are also abstract, which implies they have no implementation. method is declared to be abstract by using the "abstract" keyword: public abstract boolean validate(); there's no {...} following the signature; rather, a semicolon is present. method must not have an implementation following its signature, the enclosing class must be declared to be abstract. implementation of validate() in one of the concrete derived classes: Book. package domain; public class Book extends LibrarayItem { public boolean validate() { if (id <= 0) return false; if (title == null || title.equals("")) return false; if (author == null || author.equals("")) return false; if (pages <= 0) return false; return true; } } another technique for declaring "abstract" interfaces by way of the "interface" keyword: package domain; public interface IValidator { public boolean validate(); } interface construct is used to capture zero or more method signatures without specifying their implementation details. methods enclosed within an interface definition must be "abstract" in the sense that they must not provide an implementation (i.e., {...}), but the abstract keyword is not necessary. interfaces are implemented in classes with the "implements" keyword. User and Login are not defined as abstract since implementations are provided for the validate() method. package domain; public abstract class LibraryItem implements IValidator { public abstract boolean validate(); } uniform interface for validating objects. package domain; public class User implements IValidator { public boolean validate() { } } package domain; public class Login implements IValidator { public boolean validate() { } } while Java does not support multiple class inheritance , it does support the implementation of multiple interfaces in a single class. To serialize Login objects across a secure network Login class needs to implement the Serializable interface. package domain; public class Login implements IValidator, Serializable {...} Serializable does not contain any methods, known as a Marker Interface. Marker interfaces, used to "mark" classes (and hence their objects) as "one of those," but without any additional methods. In summary, interface constructs are used to separate interface abstractions from implementation details.
Software Layers and responsibilities
* Presentation Layer - rendering the user interface, responding to user event, creating and populating domain objects, and invoking the business layer. * The Business Layer - managing workflow by coordinating the execution of services located in the service layer. * The Service Layer - service interfaces and implementations, the movement of objects in and out of the application, and the hiding of technology choices. * The Domain Layer - abstracting domain objects (i.e., the nouns) as they appear in the problem space. Data types for: user interface ->> Presentation Layer; manage use case workflow ->>Business Layer; move objects in and out of application ->>Service Layer; abstract the "nouns" of problem space ->> Domain Layer
method toString()
default behavior is to return the object's fully qualified name followed by the object's hash code. book1.toString(); //returns: Book@1ea2df3 book2.toString(); //returns: Book@1ea2df3 this information is typically not useful, it's common to override toString().
Syntax to define a method (behavior (function in C++))
returntype methodName(optional_list_of_arguments) { ...} Names of properties (variables) and methods should start with a lower case letter, using camelCase if multiple words.
Declare the data type of a variable.
Primitive types are used to declare the data types of variables where a variable is a named memory location. datatype variableName = initialvalue; The following statements illustrate the use of Java's eight primitive data types in variable declarations: char c = 'a'; Boolean succeeded = false; byte age = 0; short index = 0; int ssn = 0; long length = 0; float pi = 3.14159f; double d = 0.0; floating point literals are defaulted to a "double" data type. To compile certain types, such as float, add a suffix to the literal: float pi = 3.14159f;
Java Collection Framework
Group of interfaces, classes, and algorithms that together provide a rich set of abstractions for grouping objects. Maps - A collection whose entries are accessed by some unique identifier. Lists - A collection whose entries can be accessed with an index. Sets - A collection whose entries are guaranteed to be unique. Queues - A collection whose entries are ordered, based on some scheme.
Package
Items of related functionality, each item in the package has a unique name. Thus, a package is a namespace, where a namespace is a collection of uniquely named items. One of main packages is "java", does not contain any data types, it's used to aggregate other packages that are nested within it. java.applet java.awt java.beans java.io java.util nested package example: java.util.concurrent.locks other nested packages do contain type definitions: - java.util contains the Date class (and other classes) - java.util.concurrent contains the Semaphore class (and other classes) - java.util.concurrent.locks contains the LockSupport (and other classes)
How do you declare a CONSTANT in Java?
Using the keyword "final": final float pi = 3.14159f;
Iterable Interface
base interface for all interfaces of the Java collection framework (excluding Map). Iterable defines exactly one method, namely: Iterator iterator(); This allows you to get an Iterator for iterating over the elements of a collection. Since all other collection interfaces extend Iterable (either directly or indirectly), they all inherit the above method; that is, they all provide an iterator for iterating over their elements. For example, the Collection interface directly extends Iterable, and hence, if you have a Collection, you can get an iterator to iterate over its elements: Collection<Book> coll = getOverdueBooks(); Iterator<Book> iter = coll.iterator(); while (iter.hasNext()) { Book book = iter.next(); } Because the need to iterate over all items in a collection is a common practice, the method iterator() is placed in the base interface Iterable, and then through inheritance, it's made available to all collections that extend Collection (because Collection extends Iterable). The extension of Iterable by Collection (and then List, Set, and Queue) is known as interface inheritance.
Constructor syntax
public class Book { // properties private String title = null; private String author = null; private boolean isCheckedout = false; // constructors public Book() { } public Book(String title, String author, boolean isCheckedout) { this.title = title; this.author = author; this.isCheckedout = isCheckedout; } } Two constructors: Book(), which is known as the default constructor Book(String title, String author, boolean isCheckedout) Default constructor - typically used to initialize properties to their "null" values, unless it is done when they are declared, then there is no need to have the first constructor. The 2nd constructor above, takes the passed parameters and assigns them to the class's properties (i.e., data members) using the keyword "this".
Access Modifiers
Information hiding is achieved through the use of access modifiers, which are keywords that are used to augment the visibility of various constructs. Public Default (no modifier) Protected Private
Definition of class
Describes the properties and behavior of a prototypical object. During runtime, the application creates individual instances of classes where the instances are called objects. For example: class - Book properties - title, author behaviors - checkOut(), checkIn() Then at runtime, the application creates a separate object for each book as they are requested by users.
Class Constructors
special methods that are called when a class is created. Must have the same name as the enclosing class, and are declared with no return value (the implied return type of the constructor is the enclosing class type). public Book() {...} public Book(int id) {...} public Book(int id, String title) {...} public Book(int id, String title, String author) {...} public Book(int id, String title, String author, int pages) {...} Constructors can be overloaded. Once you define at least one constructor (whether default or not), the implicit, hidden default constructor is not provided for you. used to initialize the data members of the newly created object either by assigning default values or by using the passed-in parameters. Constructors can also invoke other constructors either in the same class or in a base class. public Book(int id) { super(id); } Above uses "super" keyword to invoke a constructor in the base class LibraryItem. Constructors are invoked with the "new" keyword
polymorphism
Method overloading is an example of polymorphism. polymorphism can be defined as "one interface with multiple behaviors." Another form of polymorphism is method overriding, which is an example of dynamic binding. Method overriding is discussed later in this topic.
Collection Interface
general abstraction for a collection of objects. It provides a handful of methods that operate over all interfaces that extend Collection. <<interface>> Collection +add(E) : boolean +addAll(Collection) : boolean +clear() : void +contains(Object) : boolean +containAll(Collection) : boolean +isEmpty() : boolean +remove(Object) : boolean +size() : int Java API does not provide any direct class implementations of interface Collection. Instead, implementations are provided for interfaces that extend Collection, namely, List, Set, and Queue. Nevertheless, the Collection interface is often used to pass collections around since it represents the base behavior of all collections. We'll now take a look at List, Set, and Queue and see how they might be used in our Library application.
Protected modifier
Can be applied to individual data members and methods (but cannot be applied to a class); when applied, it restricts access to members of the same class and any derived class; e.g., the following method, setHashCode, is only accessible to members of the LibraryItem class and any derived classes (e.g., Book). package domain; public class LibraryItem { protected void setHashCode() { } } package domain; public class Book extends LibraryItem { public LibraryItem () { setHashCode(); } }
Default (no modifier)
Can be applied to a class and/or individual data members and methods within a class; indicates that the given item is not accessible across package boundaries (i.e., it limits access to members of the same package) e.g., the following class and method are NOT accessible outside the package in which they're defined. package domain; class Book { void checkout() { } }
Method overloading
occurs when a class contains more than one method with the same name. Within a class, methods can have the same name if there's difference: The number of parameters. And/or the type parameters. And/or order of parameters. The determination of which overloaded method to invoke is a compile-time decision; hence, it is sometimes referred to as static binding. Library library = new Library(); Book book = new Book(); library.add(book); User user = new User(); library.add(user); Notice that the statement library.add (book) is clearly targeting the add (Book) method because of the parameter data type, Book. Similarly, the statement library.add(user) is clearly targeting the add(User) method because of the parameter data type, User. Since these decisions can be determined at compile time, the binding is said to be static.
static fields and methods
By default, data members and methods are "instance" members and are said to be "non static." Non static instance data members are copied to each object instance of the class, so each object has its own copy. Non static methods can reference these copied data members through an implicit object reference (but you can access it via "this" when necessary). The concept of non static instance members is illustrated below. public class Book { private String title = ""; public void setTitle(String title) { this.title = title; } } When there's a need to not copy data members to each object instance, such as constants. They can remain in the class definition. Data members and methods that are not associated with object instances are call "class" members; they are local to the class and are not affiliated with the objects. Annotated with the keyword "static": public static final int CHECKOUT_DURATION = 21; Because the above data member is a class variable, it can be accessed through the class name as follows: int duration = Book.CHECKOUT_DURATION; Methods can also be designated as static: private static int nextId = 1; public static int getNextId() { return nextId++; } Above, method getNextId() provides access to the static data member and manages its next value. IMPORTANT: "static only sees static" but "non-static sees everything" static members can only access other static members; in particular, static members cannot access non-static members. Non-static members have access to all members (both static and non-static).
The comparing method: equals(Object)
Using equals(Object): boolean areEqual = book1.equals(book2); It compares object references, compares to see if book1 and book2 reference the same Book object in memory. In the example above, book1 and book2 reference different Book objects, so the boolean variable areEqual is set to false. This one would be true: Book book1 = new Book(); Book book2 = book1; boolean areEqual = book1.equals(book2);
Inheritance
One of the 3 main pinnacles of object-oriented programming (the others being encapsulation and polymorphism). Inheritance is a means by which a class extends another class, it acquires the properties and behavior of the class that's being extended. Sometimes called generalization/specialization, also called an "is a" relationship. For example, the class declarations of Book, Audio, and Periodical are shown below: package domain; public class Book extends LibraryItem { // properties and behavior of Book go here } package domain; public class Audio extends LibraryItem { // properties and behavior of Audio go here } package domain; public class Periodical extends LibraryItem { // properties and behavior of Periodical go here } class can only extend one other class -- cannot extend multiple classes
Method Overriding
when a method in a base class is repeated (redefined) in a derived class signature of both methods must match identically override its behavior in the LibraryItem to output the item's id and title as shown below: public class LibraryItem { ... public String toString() { return "LibraryItem, id=" + id + ", title: " + title; } } can invoke toString() to get a meaningful description of the object. Also true if the object is a Book, Audio, or Periodical because they inherit LibraryItem. Object obj = new Book(1, "Catch-22", "Joseph Heller", 485); String s = obj.toString(); actual instance is a Book and Book "is a" LibraryItem which overrides toString(), the LibraryItem's toString() method is called. Example of polymorphic dynamic binding; dynamic because it's not known until runtime which toString() method is invoked; it depends on whether the method is overridden, and if so, where. Can override toString in each of the more specialized classes: Book, Audio, and Periodical: public class Book extends LibraryItem { public String toString() { return super.toString() + ", Author, =" + author; } } super.toString() in the above return statement; its purpose is to invoke the toString() method in the base class LibraryItem. So the Book's overridden toString() method calls the LibraryItem's toString() method and then appends to it the author information that's contained in the Book. This time, since Book has overridden toString(), which overrides LibraryItem.toString(), which overrides Object.toString(), it's Book.toString() that gets called, and not the ones declared in Object or LibraryItem: Object obj = new Book(1, "Catch-22", "Joseph Heller"); String s = obj.toString(); another example of method overriding, consider the method equals(Object obj) defined by the Object class. to compare the state of two objects, overriding equals(Object obj) as shown below for LibraryItem: public class LibraryItem { public boolean equals(Object obj) { if (this == obj) return true; if ( ! (obj instanceof LibraryItem)) return false; LibraryItem item = (LibraryItem)obj; if ( this.id != item.id ) return false; if ( ! this.title.equals(item.title)) return false; return true; } } @override annotation - applied to methods that are intended to override behavior defined in a base class. annotation forces the compiler to check that the annotated method truly overrides a base class method. package domain; public class LibraryItem { @Override public String equals(Object obj) { ... } @Override public String toString() { ... } }
Keyword: new
new - creates Class objects and a constructor as shown below: Book book1 = new Book(); Book book2 = new Book("SomeTitle", "SomeAuthor", false); Constructors are invoked with the "new" keyword as shown below: Book book1 = new Book(); Book book2 = new Book(1); Book book3 = new Book(1, "Gone with the Wind", "Margaret Mitchell"); Using a constructor that allows you to initialize the object with passed-in parameters. Using the default constructor, followed by the invocation of set(...) methods.
Associations
allow classes (and their objects) to form relationships with one another. These relationships facilitate object collaboration. two types of associations: "Has a" relationships: Abstracts/models the concept of containment (e.g., a Customer has an Account; or in other words, the Customer contains an Account). "Uses a" relationships: Abstracts/models the concept of using (e.g., a Workflow manager uses various services to accomplish a task [to get some work done]). "has a" relationship with another class if it has a data member of that particular type. package domain; public class User { private Account account = new Account(); } abstracting a Login class that contains the user's credentials (username, password) and associating it with the User by declaring Login data member: package domain; public class User { private Login login = new Login(); private Account account = new Account(); } Once you declare a data member as an object type, you have essentially created a "has a" relationship between the two objects. "uses a" relationship, only difference between a "has a" and "uses a" relationship is where the declaration is made. "uses a" relationships are defined within method declarations (and not as data members of the enclosing class). So for instance, suppose a particular method needs to use a Date object to timestamp a message: package domain; public class Logger { public generateLogMessage(String message) { Date date = new Date(); } } The above method instantiates a Date object locally within the scope of the method, not as a data member (at the class/object level). "uses a" relationship. So we can summarize "has a" and "uses a" relationships as follows: "Has a" relationships are realized as object declarations located at the class level (i.e., as data members). "Uses a" relationships are realized as object declarations located within the scope of a method.
Keyword: this
this - used to disambiguate the overloaded use of variable names; in particular, "this" can be used to reference a data member (defined in the class) when that name is identical to a passed parameter. assigns the input parameter, title, to the data member, title: private String title = ""; public void setTitle(String title) { this.title = title; } inside the method setTitle(String title), the passed parameter, title, hides the data member, title, defined in the class. To access to the title data member, the keyword "this" is used to indicate a data member of the object is being referenced (and not a local variable inside the method).
Inner & Anonymous Classes
Inner classes are defined within the context of another class. package domain; public class Book extends LibraryItem implements AuthorableItem, PrintedMaterial { BookReader reader = new BookReader(); // .rest of Book definition public String nextLine() { return reader.readNextLine(); } // Note that this is not a "public" class- //only an instance of Book can access the BookReader class BookReader { public BookReader() { } public String readNextLine() { // Implementation of this method. } } // BookReader } // Book Anonymous classes are also inner classes that are defined within the context of another class, but these classes do not have a name, hence the name anonymous. Anonymous inner classes are often used as event handlers in the implementation of graphical user interfaces.
Objects example illustrated
CLASS Book title: String author: String isCheckedOut: boolean checkOut(): void checkIn(): void If a user checks out 2 books, each of them are assigned the class properties: Book Book title: Call of the Wild title: Gone with the Wind author: J. London author: M. Mitchell isCheckedOut: true isCheckedOut: true class is a type, and objects are instances of those types
What is the syntax of a sequence control flow example?
base = rate * hours; taxes = base * taxRate; net = base - taxes; count++; count--;
Maps
supports the ability to add and retrieve items from a collection using a key value pair. The key (K) is the lookup identifier and the value (V) is the item being looked up. The methods for inserting/retrieving items in/out of the Map are put(...) and get(...). General accounts - For general public use. Business accounts - For commercial organizations. Nonprofit accounts - For nonprofit organizations. The above account types can be abstracted as a Java enum (enumerator): enum AccountType {general, business, nonprofit}; Each user is allowed to have one account of each type. Accounts for a given user could be contained in a Map data structure where the key for each account is one of the above enum values (general, business, or nonprofit) as shown below: Account generalAccount = new Account(); Account businessAccount = new Account(); Account nonprofitAccount = new Account(); //... Map<AccountType, Account> accountsMap = new HashMap<AccountType, Account>(); accountMap.put(AccountType.general, generalAccount); accountMap.put(AccountType.business , businessAccount); accountMap.put(AccountType.nonprofit , nonprofitAccount); Retrieval of the accounts from the map is then achieved with the key: Account generalAccount = accountMap.get(AccountType.general); Account businessAccount = accountMap.get(AccountType.business); Account nonprofitAccount = accountMap.get(AccountType.nonprofit); In summary, the key is used to place values in the map and also to retrieve values from the same map. advantage, code readily accommodates additional account types. simply add to the enum type: enum AccountType {general, business, nonprofit, government, education};
Syntax to create a String
String s1 = new String("Hello World"); Can be created and initialized without using new: String s2 = "Hello World"; Can be concatenated with the + operator: String s3 = "Hello " + "World"; Operator == compares object references (and not the object's state), and since s1, s2 and s3 reference different String objects, == returns false: (s1 == s2); // returns false (s2 == s3); // returns false (s1 == s3); // returns false String's implementation of equals(Object) examines the values of the strings, this is true: s1.equals(s2); s2.equals(s3); s1.equals(s3); Strings objects created with the "new" operator that have the same value (the same string of characters) are always "equals()" to each other, but since they are different objects, they are never "==" to each other. One last subtle point: strings created using the double quote syntax with the same value without the new operator are always = = to each other. String s4 = "Hello World"; String s5 = "Hello World"; (s4 == s5); // returns true
Constructors are special methods defined in a class that are used to initialize ___ ___.
Answer: object instances Constructors must have the same name as the class name; the class Book will have constructors named Book (must be identical). Constructs are not allowed to explicitly declare that they return a type; rather, the implicit type they return is the enclosing class type in which they're defined.
What is the syntax of a looping control flow example?
for (int i=1; i<10; i++) { ... } while (x < y) { ... } do { ... } while (x<y)
UML Diagrams - list the behavior diagrams
Behavior Diagrams - STIACUS Sequence Timing Interactive Activity Communication Use Case State
Objects...
are instances (instantiations) of classes, as objects are created, the properties that are defined in the class template are copied to each object instance and assigned values. Thus, each object has its own copy of the class properties
String class
Java's String class is used to represent a sequence (i.e., string) of characters. A simplified class diagram of String is shown below: STRING count : int value : char[] String() String(orig:String) length : int charAt(index:int) : char concat(st : String) : String equals(obj : String) : boolean indexOf(str : String) : int isEmpty() : boolean lastIndexOf(str : String) : int length() : int replace(oldChar : char , newChar : char) : String substring(beginIndex : int, endIndex : int) : String toLowerCase() : String toUpperCase() : String toString : String * Extends class Object and thus inherits Object's properties and behavior, including method equals(Object); * Encapsulates private fields (e.g., count and value), prevents direct access to these fields by other objects; indirect access is provided through public methods; e.g., the method length() provides access to the count field; * Has two overloaded constructors, String() and String(String s). The first constructor takes no parameters, and the second takes a String object that is copied into the new String object; * Overrides methods toString() and equals() inherited from class Object by providing its own implementations; * Exhibits polymorphism (one interface, multiple behaviors) with methods toString() and equals();
Application defined classes
classes that you create for your specific application Login -username : String -password : String +setUsername : (username : String) +getUsername () : String +setPassword : (password : String) +getPassword () : String +validate() : boolean +equals(Login Login) : boolean two private data members: username and password; the minus sign means the properties are hidden in the class, cannot access them outside of class. When a class does not explicitly provide a constructor, Java implicitly provides a default constructor (one that takes no arguments). The default constructor is available for use even though it's not shown.
Importing packages
when classes are in defined in different packages. In order for ClassA, in packageA, to have visibility to ClassB, in packageB, one of three things must happen; either: - the fully qualified name of ClassB must be used, - fully qualified name of ClassB be must be imported with an import statement, - entire contents of packageB must be imported with an import statement. first technique is to use the FQN of the class. The FQN of ClassB is packageb.ClassB. to declare: package packagea; public class ClassA { packageb.ClassB b = new packageb.ClassB(); } better technique is to use an "import" statement: package packagea; import packageb.ClassB; public class A { ClassB b = new ClassB(); } import statements must appear after the package statement, and before the data type (class, interface) definition more efficient technique is to import all the data types from a given package using the * notation: import packageb.*; if multiple packages have classes with the same name, use long way (FQN) of importing package: package1.ClassXYZ xyz = new package1.ClassXYZ()
Section 22
(50 cards)
Method overriding
Subclass method can override methods from abstract or concrete classes. Within a subclass an override method in the super can still be called with super.method().
Dynamic binding
the subclass' behaviour will occur even though the client does not know it is using the subclass object.
Overriding*
Overriding and overloading support polymorphism. - Abstract class (or concrete class) implement method - subclass with same signature override superclass method. - JVM begins at bottom of type hierarchy and searches up until match found to determine which method to run. - within subclass can call superclass method with super.method()
Local variables (field)
Defined inside methods, constructors or blocks. Initialized within method and destroyed when method complete.
Java portability
Java source code (.java) > Java compiler > Java bytecode program (.class or .jar) > OS Just In Time (JIT) compiler > OS machine code
Applying Encapsulation*
- Make fields private - make accessors (getters) and mutators (setters) public - make helper (utility) methods private
this.method()
calls an instance method in the current class.
Encapsulation*
- data hiding - make fields in class private, accessible via public methods - protective barrier against outside code - access is controlled by interface - Alter code without affecting those that use it
Throwing & Catching Exceptions
Exceptions can be thrown either by the Java API or by your code. To throw an exception examples: throw new Exception(); throw new Exception("Some message goes here"); throw new LoginFailedException("User not authenticated"); Once an exception is thrown, the JVM starts looking for a handler in the call stack to transfer execution of the program. Exception handlers are declared in the context of a try/catch block. Catching Exceptions Java supports the catching of exceptions in what's known as a try/catch block or also known as a try/catch/finally block. Syntax for a try/catch block: try { ... } catch (SomeException e) { ... } The interpretation of the try/catch block is the following: the statements between the try and catch are executed sequentially, and if no exception occurs, then execution continues with the first statement that follows the try/catch block. On the other hand, if an exception occurs while executing the statements between the try/catch, execution transfers to the catch block, and then once the catch block finishes executing, execution continues with the first statement after the try/catch block.
List Interface
represents an ordered sequence of objects. That is, elements are added to the list in a particular order, and the list maintains that order. suppose we have a List, booksCheckedIn, which contains all the books that are checked-in on a given day. Then, if we add books to the list as they are checked-in, the list will maintain the order in which they were checked-in. private List booksCheckedIn = new ArrayList(); public void checkin(Book book) { booksCheckedIn.add(book); } Implementations of the List interface include ArrayList, LinkedList, Stack, and Vector. List collections do not check for duplicate entries, so if an object is added multiple times, it will appear in the list multiple times (i.e., have multiple entries). Therefore, if it's important for a given object to appear only once in the collection, the Set interface should be used as explained below.
char
16 bit unicode character
Inheritance*
Process where one object acquires the properties of another.
short
16 bit, useful if memory an issue
byte
8 bit, useful if memory an issue
extends
Long
unsigned long
System.out.println("Hello")
Prints Hello to console on a new line.
JSE API
Java Standard Edition Application Programming Interface Libraries
int
32 bit
Java
- object oriented - architecture neutral - portable - fast -secure -network aware - syntax based on C and C++
this
Denotes current object. Resolves ambiguity problems. this.tree = tree;
Queue Interface
represents an ordered collection based on some known order: e.g., first in first out (FIFO), last in first out (LIFO, a.k.a. a stack), priority. A subset of the Queue API is shown below (not including the methods inherited from interface Collection). <<interface>> Queue +element() : E +offer(E) : boolean +peek() : E +poll() : E +remove() : E order of the queue is determined by the implementing class. Classes that implement the Queue interface include LinkedList and PriorityQueue. As an example, suppose the Library system solicits user suggestions, and these suggestions need to be processed in a FIFO basis. private Queue suggestions = new LinkedList(); public void addSuggestion(Suggestion suggestion) { suggestions.add(suggestion); } The above method adds the new Suggestion to the Queue. Because the implementing class is a LinkedList, each suggestion is added to the end of the queue.
Java Runtime Environment (JRE)
Interpreter for compiled Java byte code programs. = Java Virtual Machine (JVM)
Scanner keyboard = new Scanner(System.in); x = keyboard.nextInt(); y = keyboard.nextLine();
Use scanner for keyboard input. nextInt() scans in next token as an integer nextLine() scans in current line
Defining Exceptions
exceptions are classes that extend the class Exception either directly or indirectly. For example, just a few of the many exceptions defined by the Java API. public class ClassNotFoundException extends Exception {...} public class IOException extends Exception {...} public class NullPointerException extends Exception {...} public class NumberFormatException extends Exception {...} The Exception class, most of its behavior is inherited from a base class named Throwable, which defines a message (String) property and a corresponding accessor, getMessage(). defining a custom exception that denotes a failed login: public class LoginFailedException extends Exception { public LoginFailedException() { super(); } public LoginFailedException(String msg) { super(msg); } } when defining your own exceptions, provide at least two constructors, default constructor and a constructor that takes a message string as a parameter. In both cases, the base constructor of Exception is invoked using the keyword super.
Abstract Data Types
Model of a data structure e.g. Stack(). Describes the behaviour of the data type.
Boolean
true or false
BufferedReader stdin = new BufferedReader(new InputStreamReader(system.in)) input = stdin.readLine();
Create BufferedReader instance containing a InputStreamReader instance with byte stream as input.
Abstraction (extend)
- Segregation of implementation from interface. - cannot be instantiated - Partial implementation - Class can only extend one abstract class
Interfaces (implement)
- Abstract methods. All methods are public. - Cannot be instantiated. - Class that 'implements' an interface inherits its abstract methods.
Exception Handling
events (e.g., errors) that disrupt the normal flow of execution. Examples of software exceptions: - Dividing by zero. - Using an unassigned object reference (that points to null) to invoke a method. - Failure to open/read/write/close a file. - Failure to open/read/write/close a socket connection. - Database errors. when a program executes, described as a call stack, ordered list of the active methods called up to the current point in time. stack starts with the main entry point and documents the active methods that are currently under call. once an exception occurs, it must be handled, or the program aborts. the JVM marches through the call stack looking for the nearest handler for the exception. If one is found, control is transferred. Otherwise, the program terminates.
Java Development Kit (JDK)
Java compiler and interpreter. includes JRE and JSE API
Program
set of classes, one with a 'main' method (.java)
this()
invokes the constructor.
Protected
Accessible if in same package.
Class variables
Declared with 'static' keyword. In class but not in methods. One per class - if it is set to 5 and an object changes it to 6 it will then be changed to 6 for all instances of the class.
Package
set of classes with a shared scope, typically providing a single coherent 'service' e.g. FilmFinder package
Set Interface
represents a collection of objects with no duplicates. By default, duplicates are identified by their equals(Object) method whose default behavior is defined in the root class Object. In other words, if equals(Object) is not overridden, object references will be used to determine if list entries are equal. If this behavior is not desired, equals(Object) needs to be overridden in the class for which objects are added to the set. A portion of the Set API is shown below. <<interface>> set +add(E) : boolean +addAll(Collection) : boolean +clear() : void +contains(Object) : boolean +containAll(Collection) : boolean +isEmpty() : boolean +remove(Object) : boolean +size() : int suppose the Library system has a business rule that prevents users from checking out more than one copy of a given book. During the checkout process, the user's books could be added to a Set collection where duplicate books are not added to the set. private Set booksCheckedOut = new HashSet(); public boolean checkout(Book book) { return booksCheckedOut.add(book); } The above method returns true if the book is not already in the set; otherwise, it returns false. If false is returned, the user would be notified that the duplicate book could not be checked out. Implementations of the Set interface include HashSet and TreeSet. The order of elements in a Set depends on the underlying implementation, and thus, will vary across implementations. If order is important, the interface SortedList should be used.
this.field
accesses an instance variable in the current class
Polymorphism*
Able to perform operations without knowing the subclass of the object (just superclass). Apply the same operation on values of different types as long as they have common ancestor. * parent class reference is used to refer to a child class object - program decides which to run at run time
static
means only one copy exists of the field or method and it belongs to the class not an instance. Its shared between all instances of the class.
Declaring Exceptions
required that if a method throws an exception without catching it, the exception must be declared in the method signature to notify users. method that authenticates logins, throws an exception named LoginFailedException without catching it: public bool authenticate(Login login) throws LoginFailedException {...} Any exception that occurs in a method must either be handled (caught) by that method or be declared in its signature as "throws" (see above). method throws (and does not handle) multiple exception types, instead of listing all the exceptions in the method signature, the base exception class Exception can be listed: public bool authenticate(Login login) throws Exception {...} declaration covers all possible exception types, and thus prevents you from having to list the individual exceptions when multiple exceptions can be thrown.
Does Java have multiple inheritance?*
No. Instead deal with interfaces.
Integer
unsigned int, 0 -> 2^31 -1
long
64 bit
Public
Accessible by all.
Benefits of Encapsulation*
Maintainability, Flexibility and Extensibility.
Instance variables
- variables inside class but not in methods - instantiated when class loaded - accessed by any method in class
Private
only accessible if in same class.
OO definitions
Programming paradigm that views programs as cooperating objects, rather than sequences of procedures Focuses on design of abstract data types, which define possible states and operations on variables of that type
Section 23
(50 cards)
Benefits of Refactoring
- Reduces maintenance problems - Reduces probability of errors - Reduces duplicated code.
Methods rule (subtypes)
Calls to the subtype methods must behave like calls to the corresponding supertype method.
Maps
- maps keys to values - can have duplicate keys - each key maps to one value
Interface rules
1. Exceptions should be declared by interface method. 2. Signature and return type should be the same for interface and implemented class. 3. A class can implement multiple interfaces. 4. An interface can extend another interface.
final
- prevents class from being extended or overridden - prevents variables from being altered = constant
HashSet
data is stored in a table with an associated hash code. Has code is used as an index. Contain NO duplicates, elements kept in order.
Bounded generic types
restrict the actual type parameter allowed and can be specified by saying which class the allowed types extend. - effect = permits subtypes of the class. can have: - object of subtype B where object of its supertype A is expected.
Software Patterns
- Reusable solution to common problem. - Description/Template
Glass box
Testing code (can see code)
TDD Mantra
1. Create new tests that fail (RED) 2. Write code to pass the test (GREEN) 3. Refactor
Dummies
tests objects that are never used but exist only to satisfy syntactic requirements.
Refactor - replace error code with exception
A method returns a special code to indicate an error - throw an exception instead.
iterator()
Iterator<String> iter = names.iterator(); while(iter.hasNext()){ System.out.println(iter.next()); }
Glass box steps
Ensure 1. Statement coverage (every statement is executed at least once. 2. Branch coverage (each condition is evaluated at least once). 3. Path coverage (all control flow paths are executed at least once).
Types of Lists
ArrayList: resizable array LinkedList: FIFO Stack: LIFO Vector: deprecated
Types of sets
HashSet<E>: unordered TreeSet<E>: ordered based on value LinkedHashSet<E> : insertion ordered
Basic Enum class
public enum Level { HIGH, MEDIUM, LOW } //Assign it Level level = Level.HIGH;
Lists
- ordered sequence of elements - indexable by individual elements or range - elements can be insereted at or removed from any position - can have duplicates
Pass by value
- Primitive types - makes a copy so does not affect original
Observable
Use instead of Subject interface for Observer pattern, Observable aware of its status and it's Observers unlike Subject.
Sets
- No Duplicates - Usually unordered
Test Driven Development (TDD)
1. Tests developed before code is written. 2. Once code passes, new tests added 3. Code extended to pass new tests and refactored if necessary. Create tests you know will fail because you know implementation not in place.
Collections
- Compound data type that groups multiple elements of the same type together. - Encapsulates the data structure needed to store the elements and the algorithms needed to manipulate them. - Group of elements of the same type
Type parameter
<X> ...of type X. <String> is of type String. <Tree> is a Tree object
Refactoring Techniques
1. Extract Method 2. Extract Class 3. Rename 4. Move class 5. Replace conditional with Polymorphism 6. Decompose conditional 7. Encapsulate collection 8. Replace error code with exception 9. Split Loop
Enum
enumerated type of class. - list of constants. - use when need predefined list of values that do not represent values of textual data
Properties rule (subtypes)
Subtypes must preserve all of the provable properties of the supertype. Anything provable about A is provable about B.
Black box
Testing to specifications (code unseen)
Types of Queues
ArrayDeque: double ended ArrayBlockingQueue: fixed capacity, FIFO PriorityQueue: ordered based on value
Hamcrest
framework to help write JUnit test cases. assertThat("HelloClass", is(equalTo(Greeter.greeting())))
Assertion code
assert boolean_expression; assert boolean_expression : displayable_expression;
Refactor - split loop
You have a loop doing two things - duplicate the loop. for(Tree tree : trees) // do x // do y Will be separated into two loops, one doing x and the other doing y.
Observer Pattern
- Synchronises state and/or responses across a collection of objects in response to an event. - Have Observer and Subject - Observer queries Subject to determine what was the change of state and responds appropriately based on Subjects new state. - Subject adds observers, sets state and gets state.
Subtype rules
1. Signature rule 2. Methods rule 3. Properties rule
Types of Maps
HashMap: unordered, use for max speed TreeMap: ordered by key LinkedHashMap: inseretion ordered.
Arrays
are NOT collections. Do NOT: 1. support iterators 2. grow in size at runtime 3. are not threadsafe Primitive types of a fixed size.
3 different groups of patterns
1. Creational: managing the creation of objects 2. Structural: provide a more workable interface for client code. 3. Behavioural: managing common interactions
Fakes
test objects with working methods but have limited functionality.
Stubs
test objects whose methods return fixed values and support specific test cases only.
Signature rule (subtypes)
Subtypes must have signature-compatible methods for all of the supertypes methods. ArrayList<String> is a subtype of List<String>
Generics
- accept another type as a parameter - collections <E> <X>
Assertions
Assert that particular state must be true for program to proceed. Used for defensive programming and can be basis of formal programming proofs.
Refactor - extract method
You have a code fragment that can be grouped together: turn the fragment into a method with a self-explanatory name.
Mocking
Mocks objects that exist outside the program. Use for TDD.
Refactor - decompose conditional
Complicated if-then-else: extract methods from the condition, then part, then else parts. if (date.before(SUMMER_START)) charge = quantity * winterRate + winterService Charge; else charge quantity * summerRate BECOMES if(notSummer(date)) charge = winterCharge(quantity) else charge = summerCharge(quantity)
Queues
- ordered sequence of elements - access via endpoints - structures for LIFO and FIFO
Refactor - encapsulate collection
A method returns a collection - make it return a read-only view and provide add/remove methods. (get, set, add, remove..)
Pass by reference
- Objects and arrays - Changes affect original as it is given access to the address not just a copy.
Black box Steps
1. Considered input and output values 2. Partition inputs and outputs into ranges 3. For each equivalence class produce a) set of typical test that cover normal cases b) set of boundary value tests at or around the extreme limits of the equivalence class
Refactoring
Cleans code whilst preserving behaviour.
Section 24
(50 cards)
The Lock (version control)
- Lock-modify-unlock - When Harry accesses file he LOCKS it, reads/edits then releases lock. Sally then LOCKS it and does the same. - Only 1 person can access file at time!
Local Repository
- Stored on local computer. - Individual checks out version from Version database.
GIT
- Distributed version control. - Snapshot based = takes snapshot of each version. - Uses checksums to identify changes to files.
Version control principles
1. The Lock 2. The Merge
BoxLayout
Components placed in box. Can be aligned left, centre, right, top, middle, bottom etc.
Working directory
Checkout of the latest version of a project. May contain modified files that are not staged or committed. (Local)
BorderLayout
Splits area into 5 predefined spaces: South, north, west, east and center
Events
Every action creates an event on Event Dispatch Thread. Actions = button click, mouse over etc.
FlowLayout
Place components in the window from left to right, top to bottom.
Thread Life Cycle
new process > WAITING > RUNNING > BLOCKED - waiting starts and runs or times out - runs unless blocked until unblocked or until complete
Example of Event Listener and Event Handler
public class ButtonListener implements ActionListener { public void actionPerformed(ActionEvent e) { System.out.println(e); } } // Listens for button action, if action occurs then will print the action.
GridBagLayout
Components placed in a grid. - given distinct sizes (# rows and # columns assigned to component. Most common for sophisticated GUIs.
JInternalFrame
Use for Multiple Document Interface (MDI)
Central Repository
- Version database stored on server. - Team checks out version from database. - Checkout = make local copy.
Threads
- Multiple processes within a single program. - Executed in slices or parallel. - Based on a priority system : this thread before that thread - Separate units of a parent process that share the resources of the parent process
Hashcode (GIT)
Uniquely identify each commit.
Checkout
- Make a local copy of code/file. - Checkout from Version Database.
Abstract Windowing Toolkit (AWT)
Java GUI programming toolkit - native widgets - heavyweight components - small range of widgets - look and feel can not be changed. - Dependent on platform. OLD.
Event loop
1. Action (button click, mouse movement...) registered generating event that can be tracked 2. Event passed to affected component, 3. Listen for Event 4. Respond to Event.
git commit
Commits staged files to Repo.
JDesktopPane
Use to create a virtual desktop or multiple-document interface. Use as main frame when you want to have internal frame.
git add
Tells git to track files. Tells git to stage tracked files that have been modified. Recursive.
The Merge (version control)
- Copy-modify-merge - Harry and Sally access at same time, both edit, Sally publishes first and Harry receives 'out-of-date' error. - Version control system performs diff operation and identifies conflicts. - Manual intervention required if conflicts. - Harry has to compare latest version with his own, creates merge and publishes merge.
Disadvantages of the the Lock (version control)
- Unnecessary serialisation of development steps - Programmers forget to release the lock - Doesn't consider dependencies between classes that may be out to different people at the same time.
Event Adapters
- Adapter = Design Pattern - Each Listener has a corresponding Adapter abstract class
Distributed version control
Stored local, everyone has a copy of the entire repository and its history. - very low latency for file inspection, copy and comparisons.
Top Swing Level
- Can appear anywhere on desktop. - Serve as root of containment hierarchy, all others must be contained in top lvl container - have content pane - can have menu bar added JFrame, JDialog, JApplet
Intermediate Swing Level
Components used for laying out the UI (User Interface). JPanel, JInternalFrame, JScrollPane, JSplitPane, JTabbedPane (e.g. chrome browser has tabs at top)
GridLayout
all components are placed in a table like structure and are the same size.
4 heavyweight Swing components
1. JFrame 2. JWindow 3. JDialog 4. JApplet
Event Listeners (Observer?)
Listen for events in components. Instances of Listener classes. Provided in interfaces.
Event Handlers
Methods that host the response code (tells what to do if event occurs).
Disadvantages of TDD
- difficult for programs with complex interactions with their environment e.g. GUI - Bad tests = bad code - if programmer write own code = blind spots - scalability debated.
git init
Initialise an empty repository
Danger of using GIT
Only one repository.
JPanel
- Contain parts of UI - Can contain other JPanels - Contain multiple different types of components.
JTextArea
Display textual data or accept large amounts of text input.
Atomic Swing Components
- Basic Controls JButton, JComboBox, JList, JMenu, JSlider, JSpinner, JTextField - Uneditable Displays JLabel, JProgressBar - Interactive Displays JColorChooser, JFileChooser, JTable, JTextArea, JTree
Swing
Java GUI programming toolkit - Native code windows - 4 heavyweight components - lightweight - look and feel can be changed (can be set or depend on platform) - wide range of widgets
JTextField
Hold any text. Can be editable. Use for small amounts of input.
Head pointer (GIT)
Points to the current branch.
Commit
Publishes file to GIT directory (Repository). Making it accessible by team. Use 'git commit -m "Message"'.
git push
Publishes committed local files to remote repo.
GUI
Graphical User Interface
Staging area
File indicating the modified files in the working directory to be included in the next commit. (Local). Use 'git add'.
Close Window (GUI)
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JComboBox
Combine text field and drop down list. Text field can be editable.
Mocks
test objects that know how they are meant to be used.
Dangers of threads
Introduce ASYNCHRONOUS BEHAVIOUR - multiple threads access same variable/storage location can become unpredictable. - only one should alter data at a time.
Disadvantages of Centralised Servers
- Central point of failure - Latency if remote - Inaccessibility if off-line
Section 25
(50 cards)
Target (ANT)
- can depend on other targets - 'depends' specifies order targets should be executed.
Mismanaged threads result in
1. Race Conditions 2. Deadlock 3. Starvation 4. Livelock
executeUpdate (SQL command)
Returns number of rows affected. Use for INSERT, UPDATE or DELETE. rows = st.executeUpdate("UPDATE names SET age = 21 WHERE name = 'John Howard'");
java.lang.Runnable
- implemented by classes that run on threads - implement instead of extending Thread Need a Thread wrapper - Thread one = new Thread(new HelloRunnable());
Continuous Integration (CI)
Build generated whenever: - change to a defined repository - fixed intervals e.g. CruiseControl, Jenkins (previously Hudson) - configured using config.xml file - effectiveness depends on quality of system integration tests. limited by GUI's etc. Use mocks etc to overcome issues. http://www.thoughtworks.com/continuous-integration
Task (ANT)
Piece of code that can be executed - can have multiple attributes (arguments) name: name of task <name attribute1="value1" attribute2="value2"...>
Depend (ANT)
determines which classfiles are out of date with respect to their source.
CallableStatement (JDBC)
Relies on a stored procedure. Can contain ? placeholders. public static final String INSERT_SQL = "call addName(?, ?)"; insert = connection.prepareCall(INSERT_SQL); ResultSet rs = select.executeQuery(); insert.setString(1, "Zaphod Beeblebrox"); insert.setInt(2, 26); insert.executeUpdate();
Connection interface (JDBC)
Connection to DB through connection object Uses Singleton pattern = only one instance of connection. connection = DBConnection.getInstance(); try { Statement st = connection.createStatement();
Checked exception
- if can throw 'checked' exception must: 1. include exception handling code to catch exception or 2. Declare to its caller that it potentially throws exception
Object lock (synchronization)
- must be final, otherwise could change and dif threads synchronize on dif objects private final static Object sharedLock = new Object(); public static void displayMessage(JumbledMessage jm) throws InterruptedException { synchronized(sharedLock) { for (int i = 0; i < jm.message.length(); i++) { System.out.print(jm.message.charAt(i)); Thread.currentThread().sleep(50); } System.out.println();}}
PreparedStatement (JDBC)
Precompiled SQL statements at object creation time rather than execution time. - Use ? to indicate parameters for the statement and fill in later. public static final String SELECT_SQL = "SELECT * FROM names ORDER BY age"; select = connection.prepareStatement(SELECT_SQL); ResultSet rs = select.executeQuery(); insert.setString(1, "Zaphod Beeblebrox"); insert.setInt(2, 26); insert.executeUpdate(); // So 1 is names and 2 is age.
Race Conditions
- Concurrent access problem - outcome depends on which thread gets to a value first - Results in asynchronous changes = can't predict variables affected by threads - Shared variables need 'volatile' keyword
Levels of synchronization
1. None 2. Class methods only (ensure class variables thread safe) 3. Class and Instance methods - protect integrity of all fields - allow different threads share objects - all fields protected from simultaneous access and modification
Static Variables and Threads
Static Variables are NOT thread safe. - Use ThreadLocal or InheritableThreadLocal type declaration
Service Threads
or daemon threads - Contain never ending loop - Receive and handle service requests - Terminates only when program terminates. So detects when all non-daemons finished then kills daemons and programs terminate.
Deadlock
- Blocked waiting for a resource held by another blocked thread - all philosophers pick up the left fork at the same time before any can pick up the right fork - No progress possible.
notify() (Thread Communication)
sends wake up call to a single blocked thread. Cannot specify which thread to wake.
JDBC (Java Database Connectivity)
Used to interface DBMS and Java code. - Gives database vendor independence. - Java gives platform dependence Allows: 1. Making connection to database 2. Creating SQL or MySQL statements 3. Executing SQL or MySQL queries to database 4. Viewing and modifying the resulting records.
Thread safety
- for GUIs use EventDispatch thread and SwingUtilities.invokeLater(new threadClass()) public static void main(String[] args) { JFrame.setDefaultLookAndFeelDecorated(true); SwingUtilities.invokeLater(new HelloWorldSwing());
CruiseControl
Build scheduler - <listeners> listen to the status of the build - <bootstrappers> actions to do before build
Attributes (ANT)
Can contain references to a property. References resolved before task executed
Statement (JDBC)
Defines methods that enable send SQL commands and receive data from DB. Statement st = connection.createStatement(); connection.execute(); etc.
Client/server programming (Daemon thread)
1. Server listens using daemon thread for each port 2. Request comes, daemon thread responds 3. Creates/calls another thread to do work 4. Worker thread terminates naturally 5. Daemon thread continues monitoring port
Exceptions
undesireable events outside the 'normal' behaviour of a program. - recoverable (not always) - handled in method where occur or propagated to calling program
Livelock
- Thread acts in response to another thread, makes no progress but is not blocked - Thread states can and do change but no progress can be made - think of corridor dance example Philosophers pass fork back and forth but no one gets to eat.
join (Threads)
Makes other thread/next thread wait until the first thread terminated. one.start(); two.start(); one.join(); three.start(); // Three has to wait on one
In the Swing GUI framework use event handlers to respond to GUI based actions. Case of component responding to button click, models works by requiring:
That the component implement the actionListener interface, that the JButton add the component as an active listener in its collection and that the component provide suitable responses to be called from within the actionPerformed method once the event is triggered.
ResultSet (JDBC)
Used to store the result of an SQL query. // get all current entries ResultSet rs = select.executeQuery(); // use metadata to get the number of columns int columnCount = rs.getMetaData().getColumnCount(); // output each row while (rs.next()) { for (int i = 0; i < columnCount; i++) { System.out.println(rs.getString(i + 1)); }
notifyAll() (Thread Communication)
sends wake up call to all blocked threads - if called when thread does not have lock = Illegal MonitorStateExpectation
execute (SQL command)
Returns boolean use for RENAME, DROP TABLE etc. st.execute("DROP TABLE names");
Starvation
- Some threads allowed to be greedy and others starved of resources, making no progress So some of the philosophers eat everything others get nothing.
Thread communication
Multiple threads notify each other when conditions change, when to wait or stop waiting. - void wait(), wait(long time), wait(long time, int nano) - void notify() - void notifyAll()
Class lock (synchronization)
public static synchronized void displayMessage(JumbledMessage jm) throws InterruptedException { for (int i = 0; i < jm.message.length(); i++) { System.out.print(jm.message.charAt(i)); Thread.currentThread().sleep(50); } System.out.println();}
Synchronization
- Only allows one thread access at a time. - Sets a lock on the method or object - When synchronized method terminated, lock released - Places an overhead on execution time (Don't over use it)
Thread safe variables
Variables that belong to only one thread. - Local variables are thread safe! - Instance and static variables are not thread safe
wait() (Thread Communication)
Puts current thread into blocked state. - can only call from method that has a lock (is synchronized) - Releases lock - Either waits specified time or until notified by another thread. - Best to put in while loop. - useful when all threads
Unchecked exception
- errors and run time exceptions - result of programming errors AssertionError, VirtualMachineError, ArithmeticException, NullPointerException, IndexOutOfBoundsException
Safely stop thread
- control loop with stopFlag - make sure stopFlag independent of other threads - check value frequently - use end() private volatile boolean stopflag; stopflag = false; //in run() try block: while (!stopflag) { d = Math.log(i++); sleep(1);} public void end() { stopflag = true;}
Batch processing (JDBC)
Reduce overhead by executing a large number at once.
volatile
- thread goes straight to source and doesn't locally ache - ensuring working with up to date data "Go back and check" - reduce concurrent thread access issues. private volatile boolean pleaseStop; while (!pleaseStop) { // do some stuff... }
run()
main method for an individual thread
ANT
- specifies how to build something. - require configuration file build.xml - A depends on B means that B needs to occur before A. - if B compiled then A will compile immediately.
Transaction processing (JDBC)
e.g. Funds transfer commit() rollback() savepoint..
Advantages of exceptions
- error handling is separated from normal code - allows different kind of errors to be distinguished
Benefits of Continuous Integration
- allow team detect problems early - reduce integration problems - increase visibility
executeQuery (SQL command)
Returns ResultSet. Use for SELECT. ResultSet rs = st.executeQuery("SELECT * FROM names")
Maven
java.lang.Thread
provides infrastructure for multithreaded programs. can extend thread but issue with multiple class inheritance so extend runnable instead.
Timer
- implements Runnable - runs a TimerTask object on JVM background thread - so use to Schedule Tasks (threads) // set Timer as a daemon thread Timer timer = new Timer(true); timer.schedule(hello1, 1000); // will run after 1000 milisec timer.schedule(hello2, 2000);
Section 26
(50 cards)
Assuming a=3, the values of a and b after the assignment b=a-- are _____.
2, 3
Assuming c=5, the value of variable d after the assignment d=c * ++c is _____.
30
String Concatenation Operator
The name for a plus (+) operator that combines its two operands (Strings) into a single String.
The _ component allows users to add and view multiline text.
JTextArea
Throwable
Used to define own exceptions.
setEditable
Sets the editable property of the JTextArea component to specify whether or not the user can edit the text in the JTextArea.
Users cannot edit the text in a JTextArea if its _ property is set to false.
Editable
The UML decision/merge symbols can be distinguished by _.
The number of flowlines entering or exiting the symbol and whether or not they have guard conditions. (Both of the above)
BLOB
Binary Large OBject - converted to byte arrays before being stored - important in cloud services - data that doesn't fit in standard data types e.g. pictures, media files, archives - alternative to serialization
JTextArea
Control that allows the user to view multiline text.
Postincrement
counter++
A _ is a variable that helps control the number of times that a set of statements will execute.
Counter
Try catch exception block
try {... normal code} catch(exception-class object) {... exception-handling code}
How many JCheckBoxes in a GUI can be selected at once?
Any number
-- (unary decrement operator)
Subtracts one from an integer variable.
CLOB
Character Large OBject
JTextArea method _ adds text to a JTextArea.
Append
Preincrement
++counter
++ (unary increment operator)
Adds one to an integer variable.
Counter
A variable often used to determine the number of times a block of statements in a loop will execute.
Escape Character
This character (\) allows you to form escape sequences.
Diamond (in the UML)
The UML symbol that represents the decision symbol or the merge symbol, depending on how it is used.
The UML represents both the merge symbol and the decision symbol as _____.
Diamonds
Infinite Loop
A logical error in which a repetition statement never terminates.
A(n) _ loop occurs when a condition in a while statement never becomes false.
Infinite
Counter-controlled repetition is also called _.
Definite repetition
Decrementing
The process of subtracting 1 from an integer.
Append
A method that adds text to a JTextArea component.
Counter-Controlled Repetition (Definite Repetition)
A technique that uses a counter to limit the number of times that a block of statements will execute.
Header
A line of text at the top of a JTextArea that clarifies the information being displayed.
Merge Symbol
A symbol in the UML that joins two flows of activity into one flow of activity.
Suite of tests
Run multiple test classes as a single test @RunWith(Suite.class) @Suite.SuiteClasses({class_name, ...})
While Statement
A specific control statement that executes a set of body statements while a loop continuation condition is true.
The body of a while statement executes _____.
If it's condition is true
The text that appears alongside a JCheckBox is referred to as the ____.
JCheckBox Text
Predecrement
--counter
JTextArea method _, when called with an empty string, can be used to delete all the text in a JTextArea.
setText
Loop-Continuation Condition
A condition used in a repetition statement (such as while) that repeats only while the statement is true.
Counter-controlled repetition is also called _____ because because the number of repetitions is known before the loop begins executing.
Definite repetition
Loop
Another general name for a repetition statement.
Postdecrement
counter--
The _ statement executes until its loop-continuation condition becomes false.
While
The line of text that is added to a JTextArea to clarify the information that will be displayed in tabular format is called a _____.
Header
@Test(timeout = T)
Test fails if runs longer than T milliseconds.
@Test(expected = X.class)
Test is expected to throw exception X.
Escape Sequence
The combination of a backslash and a character that can represent a special character, such as a newline or a tab.
Incrementing
The process of adding 1 to an integer.
In a UML activity diagram, a(n) _ symbol joins 2 flows of activity into 1 flow of activity.
Merge
setText
Sets the text property of the JTextArea component.
Repetition Statement
A general name for a statement that allows the programmer to specify that an action or actions should be repeated, depending on a condition.
Section 27
(50 cards)
2
If the variable x contains the value 5, what value will x contain after the expression x -= 3 is executed?
block
A set of statements that is enclosed in curly braces ({ and }).
The condition expression1 && expression2 evaluates to true when __.
Both expressions are true.
True and False
A variable of type boolean can be assigned the values _____.
Executable
Pseudocode usually only describes ____ lines of code.
A variable or an expression can be examined by the _____ debugger command.
Decimal, Text
Class DecimalFormat is used to control how ____ numbers are formatted as ____.
algorithm
A procedure for solving a problem, specifying the actions to be executed and the order in which these actions are to be executed.
String.equals
A method that evaluates whether a String contains specific text specified.
A JCheckBox is selected when the isSelected method returns __.
True
==, <= and <
The ______ operators return false if the left operand is greater than the right operand.
{}
The body of an if statement that contains multiple statements is placed in _____.
Double
If... else is a ____-selection statement.
The JOptionPane dialog icon typically used to caution the user against potential problems is the ____.
Exclamation point
OR operator (||)
A logical operator used to ensure that either or both of two conditions are true. Performs short-circuit evaluation.
Decimal points
The double type can be used to store numbers with ____ ____.
JOptionPane.ERROR_MESSAGE
Icon containing a stop sign, alerts the user of errors or critical situations.
Sequential
The process of application statements executing one after another in the order in which they are written is called ____ execution.
Transfer of control
A ______ occurs when an executed statement does not directly follow the previously executed statement in the written application.
logical operators
Operators (&&, ||, &, |, ^ and !) that can be used to form complex conditions by combining simple ones.
Three
All Java applications can be written in terms of ____ types of program control.
JCheckBox text
The text that appears alongside a JCheckBox.
state button
A button that can be in the on/off (true/false) state, such as a CheckBox.
Operator && __.
Performs short-circuit evaluation
An action
In an activity diagram, a rectangle with curved sides represents _____.
isSelected
Specifies whether the JCheckBox is selected (true) or deselected (false): JCheckBox._____. (method)
Multiplies
The *= operator ____ the value of its left operand by the value of the right one and store it in the left one.
NOT (!) operator
A logical operator that enables a programmer to reverse the meaning of a condition, true is false, false is true if evaluated.
set
You can modify the value of a variable by using the debugger's ____ command.
Sequence, selection and repetition
The three types of program control are _____.
showMessageDialog
Used to display a message dialog: JOptionPane.____.
Program control
____ refer(s) to the task of executing an application's statements in the correct order.
The 2nd argument passed to method JOptionPane.showMessageDialog is ____.
The message displayed by the dialog
You can examine the value of an expression by using the debugger's ____ command.
The condition Exp1 ^ exp2 evaluates to true when ____.
Either exp1 is false and exp2 is true or exp1 is true and exp2 is false.
Nesting
Placing an if...else statement inside another one is an example of ____ if-else statements.
XOR operator (^)
A logical operator that evaluates to true if and only if one operand is true.
JOptionPane.WARNING_MESSAGE
Icon containing an exclamation point, cautions the user of potential problems.
The ___ constant can be used to display an error message in a message dialog.
JOptionPane.ERROR_MESSAGE
format
Method ____ of DecimalFormat can display double values in a special format, such as with two digits to the right of the decimal point.
JOptionPane
A class that provides a method for displaying message dialogs and constants for displaying icons in those dialogs.
Pseudocode
____ is an artificial and informal language that helps programmers develop algorithms.
Constant
A variable whose value cannot be changed after its initial declaration is called a _____.
+=
The _____ operator assigns to the left operand the result of adding the left and right operands.
AND operator (&&)
A logical operator used to ensure that two conditions are both true before choosing a path of execution. Performs short-circuit evaluation.
The condition exp1 || exp2 evaluates to false when ___.
Both are false.
selected
Defines if a component has been selected. (property)
message dialog
The general name for a dialog that displays messages to users.
Algorithm
A(n) ___ is a procedure for solving a problem in terms of the actions to be executed and the order in which these actions are to be executed.
Final
Constants are declared with keyword ____.
Section 28
(50 cards)
arithmetic operators
These operators are used for performing calculations.
double
A datatype that can represent numbers with decimal points.
A debugger command that is used to examine the values of variables and expressions.
integer
A whole number, such as 919, -11 or 0 (not a decimal or fraction)
empty string ("")
A string that does not contain any characters.
double
Variable type that is used to store floating-point numbers.
if statement
This single-selection statement performs action(s) based on a condition.
final
Keyword that precedes the data type in a declaration of a constant.
logic error
An error that does not prevent the application from compiling successfully, but does cause the application to produce erroneous results when it runs.
Double.parseDouble
A method that converts a String containing a floating-point number into a double value.
workflow
The activity of a portion of a software system.
nondestructive
The process of reading from a memory location, which does not modify the value in that location.
program control
The task of executing an application's statements in the correct order.
keyPressed event
The event that occurs when any key is pressed in a JTextField.
int value
The variable type that stores integer values.
clear
Debugger command that displays a list of all the breakpoints in an application.
-g compiler option
This flag or option causes the compiler to generate debugging information that is used by the debugger to help you debug your applications.
jdb
Starts the Java debugger when typed at the Command Prompt.
DecimalFormat
The class used to format floating-point numbers (that is, numbers with decimal points).
relational operators
Operators < (less than), > (greater than), <= (less than or equal to) and >= (greater than or equal to) that compare two values.
% (remainder operator)
This operator yields the remainder after division.
destructive
The process of assigning data (or writing) to a memory location, in which the previous value is overwritten or lost.
single-selection statement
A statement, such as the if statement, that selects or ignores a single action or sequence of actions.
declaring
When you specify the type and name of a variable to be used in an application, you are ____ a variable.
floating-point number
A number with a decimal point, such as 2.3456, 0.0 and -845.4680
format
DecimalFormat method that takes a double, returns a String containing a formatted number.
declaration
Code that specifies the name and type of a variable.
primitive type
A variable type already defined in Java. (Examples: boolean, byte, char, short, int, long, float and double)
formatting
Modifying the appearance of text for display purposes.
breakpoint
A marker that can be set in the debugger at any executable line of source code, causing the application to pause when it reaches the specified line of code.
control
A program statement (such as if, if...else, switch, while, do...while or for) that specifies the flow of ____.
decision
The diamond-shaped symbol in a UML activity diagram that indicates a ____ is to be made.
pseudocode
An informal language that helps programmers develop algorithms.
if...else statement
This double-selection statement performs action(s) if a condition is true and performs different action(s) if the condition is false.
nested statement
A statement that is placed inside another control statement.
set
Debugger command that is used to change the value of a variable.
bug
A flaw in an application that prevents the application from executing correctly.
name
The ____ of a variable is used in an application to access or modify a variable's value.
asterisk (*)
An arithmetic operator that performs multiplication.
initialization value
The beginning value of a variable.
forward slash (/)
An arithmetic operator that indicates division.
assignment operator
This operator, =, copies the value of the expression on the right side into the variable.
double-selection statement
A statement, such as if...else, that selects between two different actions or sequences of actions.
equality operators
Operators == (is equal to) and != (is not equal to) that compare two values.
break mode
Debugger mode the application is in when execution stops at a breakpoint.
cont
Debugger command that resumes program execution after a breakpoint is reached while debugging.
Breakpoint
One reason to set a _____ is to be able to examine the values of variables at that point in the application's execution.
constant
A variable whose value cannot be changed after its initial declaration.
debugger
Software that allows you to monitor the execution of your applications to locate and remove logic errors.
cont
Debugger command that resumes program execution after a breakpoint is reached while debugging.
Section 29
(50 cards)
Parentheses
4. In Java, use ________ to force the order of evaluation of operators.
body
A set of statements that is enclosed in curly braces ({ and }). This is also called a block. Another name for this is a ____.
event
An action that can trigger an event handler.
run
Debugger command to begin executing an application with the Java debugger.
extending a class
Creating a new class based on an existing class (also called inheritance). This action is called ____.
class name
The identifier used as the name of a class.
Empty String
3. The ____ is represented by "" in Java.
Nondestructive
6. Reading a value from a variable is a ________ process.
Replaces
5. When a value is placed into a memory location, the value ____ the previous value in that location.
full-line comment
A comment that appears on a line by itself in source code.
comment
A ____ begins with two forward slashes (//).
setText()
4. Use the ____ method to clear any text displayed in a JTextField.
stop
Debugger command that sets a breakpoint at the specified line of executable code.
Comment
9. A breakpoint cannot be set at a(n) ____.
operator precedence
Rules of ____ ____ determine the precise order in which operators are applied in an expression.
straight-line form
The manner in which arithmetic expressions must be written so they can be typed in Java code.
size
The ____ of a variable holds the number of bytes required to store a value of the variable's type. For example, an int is stored in four bytes of memory and a double is stored in eight bytes.
Redundant
1. Parentheses that are added to an expression simply to make it easier to read are known as ________ parentheses.
Name and type
3. Every variable has a ________.
Valid Identifier
1. The name of a variable must be a ____.
int
9. Variables used to store integer values should be declared with keyword ________.
/
2. The ________ operator performs division.
actionPerformed
The kind of event that occurs when a JButton is clicked.
Primitive
2. Types already defined in Java, such as int, are known as ____ types.
unary operator
An operator with only one operand (such as + or -).
Left to right
5. If an expression contains several multiplication, division and remainder operations, they are performed from ________.
case sensitive
Identifiers with identical spelling are treated differently if the capitalization of the identifiers differs. This kind of capitalization is called _____.
Before
8. The expression to the right of the assignment operator (=) is always evaluated ____ the assignment occurs.
block
A group of code statements that are enclosed in curly braces ({ and }).
argument
Values inside of the parentheses that follow the method name in a method call are called ____(s).
extends keyword
The keyword the specifies that a class inherits data and functionality from an existing class.
At
10. When application execution suspends at a breakpoint, the next statement to be executed is the statement ____ the breakpoint.
Debugger command that displays the value of a variable when an application is stopped at a breakpoint during execution in the debugger.
dot separator
Allows programmers to call methods of a particular class or object.
end-of-line comment
A comment that appears at the end of a code line.
KeyPressed
7. Pressing a key in JTextField raises the ________ event.
comment (//)
Explanatory text that is inserted to improve an application's readability.
value of a variable
The piece of data that is stored in a variable's location in memory.
redundant parentheses
Extra parentheses used in calculations to clarify the order in which calculations are performed. Such parentheses can be removed without affecting the results of the calculations.
class keyword
The keyword used to begin a class declaration.
Stop
8. The debugger command ________ sets a breakpoint at an executable line of source code in an application.
binary operator
An operator that requires two operands.
In straight-line form
7. Arithmetic expressions in Java must be written ____ to facilitate entering expressions into the computer.
Not changed
6. When a value is read from memory, that value is ____.
event handler
Code that executes when a certain event occurs.
variable
A location in the computer's memory where a value can be stored for use by an application.
type
The ____ of a variable specifies the kind of data that can be stored in a variable and the range of values that can be stored, such as an integer holding 1, and a double holding 1.01.
class declaration
The code that defines a class, beginning with the class keyword.
truncating
When you are ____ in integer division, any fractional part of an integer division result is discarded.
10. The debugger command ________ allows you to "peek into the computer" and look at the value of a variable.
Section 30
(50 cards)
String.valueOf
A method that converts a numeric value (such as an integer) into text.
debugging
The process of locating and removing errors in an application.
left brace ({)
Denotes the beginning of a block of code.
setText
Use _____ to set the text on the face of a JButton.
input JTextField
A JTextField used to get user input.
keyword
A word that is reserved by Java. These words cannot be used as identifiers. Also called reserved word.
compiling
The process that converts a source code file (.java) into a .class file.
case sensitive
Distinguishes between uppercase and lowercase letters in code.
newline
A character that is inserted in code when you press Enter.
getText
A method that accesses (or gets) the text property of a component such as a JLabel, JTextField or a JButton.
JLabel component
A component used to describe another component. This helps users understand a component's purpose.
setBounds
The location and size of a JLabel can be specified with _____.
statement
A unit of code that performs an action and ends with a semicolon.
functionality
The tasks or actions an application can execute.
JTextField.RIGHT
Used with setHorizontalAlignment to right align the text in a JTextField.
multiline statement
A statement that is spread over multiple lines of code for readability.
.class file
The type of file that is executed by the Java Runtime Environment (JRE). A ____ file is created by compiling the application's .java file.
JTextField.LEFT
Used with setHorizontalAlignment to left align the text in a JTextField.
JTextField.CENTER
Used with setHorizontalAlignment to center align the text in a JTextField.
setText
A method that sets the text property of a component, such as a JLabel, JTextField or JButton.
left operand
An expression that appears on the left side of a binary operator.
identifier
A series of characters consisting of letters, digits and underscores used to name classes and GUI components.
setText
The text on a JLabel is specified with _____.
white space
A tab, space or newline is called a(n) _____.
background property
Property that specifies the background color of a content pane component.
reserved word
A word that is reserved for use by Java and cannot be used to create your own identifiers. Also called keyword.
editable property
The property that specifies the appearance and behavior of a JTextField (whether the user can enter text.)
semicolon (;)
Character used to terminate each statement in an application.
return
Some methods, when called, __ a value to the statement in the application that called the method. The returned value can then be used in that statement.
JButton component
A component that, when clicked, commands the application to perform an action.
font property
Property specifying font name (Times New Roman), style (Bold) and size (12) of any text.
horizontalAlignment property
The property that specifies the text alignment in a JTextField (JTextField.LEFT, JTextField.CENTER, or JTextField.RIGHT).
book-title capitalization
A style that capitalizes the first letter of each significant word in the text (for example, Calculate the Total).
horizontalAlignment property
The property that specifies how text is aligned within a JLabel is called:
sentence-style capitalization
A style that capitalizes the first letter of the first word In the text (for example, Cartons per shipment): other letters in the text are lowercase, unless they are the first letters of proper nouns.
operand
An expression that is combined with an operator (and possibly other expressions) to perform a task (such as multiplication).
content pane
The portion of a JFrame that contains the GUI components.
Integer.parseInt
Returns the integer equivalent of a String.
dir command
Command typed in a Command Prompt window to list the directory contents.
output JTextField
a JTextField used to display calculation results. The editable property of an output JTextField is set to false with setEditable.
bounds property
The property that specifies both the location and size of a component.
method
An application segment containing statements that perform a task.
right operand
An expression that appears on the right side of a binary operator.
JLabel
Component that displays a text or an image that the user cannot modify
uneditable JTextField
JTextField in which the user cannot type values or edit existing text. Such JTextFields are often used to display the results of calculations.
right brace (})
Denotes the end of a block of code.
Book-Title
You should use _____ capitalization for a JButton's name.
multiplication operator
This operator is an asterisk (*), used to ____ its two numeric operands.
JTextField component
A component that can accept user input from the keyboard or display output to the user.
inheritance
Creating a new class based on an existing class (also called extending a class). This action is called ____.
Section 31
(50 cards)
syntax error
An error that occurs when code violates the grammatical rules of a programming language.
Clock -hr: int -min: int -sec: int +Clock() +Clock(int, int, int) +setTime(int, int, int): void +getHours(): int +getMinutes(): int +getSeconds(): int +printTime(): void +incrementSeconds(): int +incrementMinutes(): int +incrementHours(): boolean +equals(Clock): boolean +makeCopy(Clock): void +getCopy(): Clock According to the UML class diagram above, which method is public and doesn't return anything? 1. getCopy() 2. setTime(int, int, int) 3. incrementHours() 4. equals(Clock)
setTime(int, int, int)
.java
5. Java source code files have the extension ____.
Javac
2. To compile an application, type the command ____ followed by the name of the file.
Accessor instance method names typically begin with the word "____" or the word "____" depending on the return type of the method. 1. set, is 2. get, can 3. set, get 0% 4. get, is
get, is
Assume that you have an ArrayList variable named a containing 4 elements, and an object named element that is the correct type to be stored in the ArrayList. Which of these statements adds the object to the end of the collection 1. a[3] = element; 2. a.add(element, 4); 3. a[4] = element; 4. a.add(element);
4. a.add(element);
setTitle
3. Use ____ to set the text that appears on a JFrame's title bar.
statement
Code that instructs the computer to perform a task. Every statement ends with a semicolon ( ; ) character. Most applications consist of many statements.
logic error
An error that does not prevent your application from compiling successfully but does cause your application to produce erroneous results.
Compiler
4. Syntax errors are found by the ____.
Clock -hr: int -min: int -sec: int +Clock() +Clock(int, int, int) +setTime(int, int, int): void +getHours(): int +getMinutes(): int +getSeconds(): int +printTime(): void +incrementSeconds(): int +incrementMinutes(): int +incrementHours(): boolean +equals(Clock): boolean +makeCopy(Clock): void +getCopy(): Clock Which of the following would be a default constructor for the class Clock shown in the figure above? 1. public Clock(0, 0, 0) { setTime(); } 2. private Clock(10){ setTime(); } 3. public Clock(0) { setTime(0); } 4. public Clock(){ setTime(0, 0, 0); }
public Clock(){ setTime(0, 0, 0); }
Size, style, and name
3. The font property controls the ____ of the font displayed by the Jlabel.
setIcon
4. Use ____ to specify the image to display on a JLabel.
Picture elements
10. Pixels are ____.
Consider the following class definition. public class Rectangle { private double length; private double width; public Rectangle() { length = 0; width = 0; } public Rectangle(double l, double w) { length = l; width = w; } public void set(double l, double w) { length = l; width = w; } public void print() { System.out.println(length + " " + width); } public double area() { return length * width; } public double perimeter() { return 2 length + 2 width; } } Which of the following statements correctly instantiate the Rectangle object myRectangle? (i) myRectangle Rectangle = new Rectangle(10, 12); (ii) class Rectangle myRectangle = new Rectangle(10, 12); (iii) Rectangle myRectangle = new Rectangle(10, 12); 1. Only (ii) 2. Only (iii) 3. Only (i) 4. Both (ii) and (iii)
Only (iii)
Components of a color
9. RGB values specify the ____.
How many constructors can a class have? 1. Any number 2. 0 3. 1 4. 2
Any number
the line number of the error; a brief description of the error.
5. Upon finding a syntax error in an application, the compiler will notify the user of an error by giving the user ____.
Title bar
The area at the top of a JFrame where its title appears.
size property
Property that specifies the width and height, in pixels, of a component.
JLabel
The component that displays text or an image that the user cannot modify.
Size and location of component
6. The bounds property controls the ____.
String literal
A sequence of characters within double quotes.
word is spelled incorrectly; parentheses is omitted.
6. Syntax errors occur for many reasons, such as when a(n) ____.
Person -name: String +setName(String name): void +getName(): String ^ Student -studentID: long +Student(String sname, long sid) +setID(): long Which of these fields or methods are inherited (and accessible) by the Student class? 1. getName(), setName(), name 2. name, getName(), setName(), getID() 3. studentID, name, getName(), setName(), getID() 4. getName(), setName(), toString() 5. None of them 6. getName(), setName(), studentID, getID()
4. getName(), setName(), toString()
.java
1. A source code file has the ____ extension.
Clock ---------------------------------------- -hr: int -min: int -sec: int --------------------------------------- +Clock() +Clock(int, int, int) +setTime(int, int, int) : void +getHours() : int +getMinutes() : int +getSeconds() : int +printTime() : void +incrementSeconds() : int +incrementMinutes() : int +incrementHours() : int +equals(Clock) : boolean +makeCopy(Clock) : void +getCopy() : Clock ---------------------------------------- According to the UML class diagram above, which of the following is a data member? 1. printTime() 2. min 3. Clock() 4. Clock
2. min
Consider the following class definition. public class Rectangle { private double length; private double width; private double area; private double perimeter; public Rectangle() { length = 0; width = 0; } public Rectangle(double l, double w) { length = l; width = w; } public void set(double l, double w) { length = l; width = w; } public void print() { System.out.println(length + " " + width); } public double area() { return length * width; } public double perimeter() { return 2 length + 2 width; } } Suppose that you have the following declaration. Rectangle bigRect = new Rectangle(10, 4); Which of the following sets of statements are valid (both syntactically and semantically) in Java? (i) bigRect.area(); bigRect.perimeter(); bigRect.print(); (ii) bigRect.area = bigRect.area(); bigRect.perimeter = bigRect.perimeter(); bigRect.print(); 1. Both (i) and (ii) 2. Only (ii) 3. Only (i) 4. None of these
Only (i)
pixel
A point on your computer screen. ____ is short for "picture element".
Compiler
2. The __________ converts a .java file to a .class file.
RGB value
The amount of red. green and blue needed to create a color.
Person -name: String +setName(String name): void +getName(): String ^ Student -studentID: long +Student(String sname, long sid) +setID(): long To access the name instance variable from inside a method defined in the Student class, name must use the _____________ modifier. (We'll ignore the default package-private access if no modifier is used.) 1. Any of them 2. public 3. protected or public 4. protected 5. private 6. private or protected
3. protected
int[] num = new int[100]; for (int i = 0; i < 50; i++) num[i] = i; num[5] = 10; num[55] = 100; What is the data type of the array above? 1. num 2. int 3. char 4. list
int
setBackground
1. Use ___ to specify the content pane's background color.
To print the last element in the array named ar, you can write : A. System.out.println(ar[length-1]); B. System.out.println(ar[length()-1]); C. System.out.println(ar[ar.length-1]); D. System.out.println(ar[ar.length]);
System.out.println(ar[ar.length-1]);
To print the last element in the array named ar, you can write : A. System.out.println(ar.length); B. System.out.println(ar[length]); C. System.out.println(ar[ar.length]); D. None of these
None of these
icon property
The property that specifies the file name of the image displayed in a JLabel.
double[][] vals = {{1.1, 1.3, 1.5}, {3.1, 3.3, 3.5}, {5.1, 5.3, 5.5}, {7.1, 7.3, 7.5}} What is the value of vals.length in the array above? 1. 16 2. 4 3. 0 4. 3
4
setHorizontalAlignment
8. Use ____ to align the text displayed inside a Jlabel.
When you override a method defined in a superclass, you must do all of these except: 1. Use exactly the same number and type of parameters as the original method 2. Use an access specifier that is at least as permissive as the superclass method 3. Change either the number, type, or order of parameters in the subclass. 4. Return exactly the same type as the original method.
3. Change either the number, type, or order of parameters in the subclass.
String
A sequence of characters that represents text information.
A method that has the signature void m(int[] ar) : A. can change the values in the array ar B. cannot change the values in the array ar C. can only change copies of each of the elements in the array ar D. None of these
can change the values in the array ar
javac command
The command that compiles a source code file (.java) into a . class file.
location property
The property that specifies where a component's upper-left corner appears on the JFrame.
semicolon ( ; )
The character used to indicate the end of a Java statement.
text property
The property that specifies the text displayed by a JLabel.
.java file
The type of file in which programmers write the Java code for an application.
Constructors have the same name as the ____. 1. data members 2. package 3. class 4. member methods
class
setText
7. A Jlabel component displays the text specified by ____.
source code file
A file with a .java extension. These files are editable by the programmer, but are not executable.
Section 32
(50 cards)
char[][] array1 = new char[15][10]; What is the value of array1.length? 1. 15 2. 0 3. 2 4. 10
15
Which of the following class definitions is correct in Java? (i) public class Student { private String name; private double gpa; private int id; public void Student() { name = ""; gpa = 0; id = 0; } public void Student(String s, double g, int i) { set(s, g, i); } public void set(String s, double g, int i) { name = s; gpa = g; id = i; } public void print() { System.out.println(name + " " + id + " " + gpa); } } (ii) public class Student { private String name; private double gpa; private int id; public Student() { name = ""; gpa = 0; id = 0; } public Student(String s, double g, int i) { set(s, g, i); } public void set(String s, double g, int i) { name = s; gpa = g; id = i; } public void print() { System.out.println(name + " " + id + " " + gpa); } } 1. Both (i) and (ii) 2. None of these 3. Only (ii) 4. Only (i)
3. Only ii
Suppose that sales is a two-dimensional array of 10 rows and 7 columns wherein each component is of the type int , and sum and j are int variables. Which of the following correctly finds the sum of the elements of the fifth row of sales? 1. sum = 0; for(j = 0; j < 10; j++) sum = sum + sales[5][j]; 2. sum = 0; for(j = 0; j < 7; j++) sum = sum + sales[4][j]; 3. sum = 0; for(j = 0; j < 10; j++) sum = sum + sales[4][j]; 4. sum = 0; for(j = 0; j < 7; j++) sum = sum + sales[5][j];
2. sum = 0; for(j = 0; j < 7; j++) sum = sum + sales[4][j];
Assume that you have an ArrayList variable named a containing many elements. You can rearrange the elements in random order by writing:
Collections.shuffle(a);
What does the following loop do? int[] a = {1, 3, 7, 0, 0, 0}; int size = 3, capacity = 6, pos = 0; a[pos] = a[size - 1]; size--;
Removes the 1 from the array, replacing it with the 7. Array now unordered.
int[] num = new int[100]; for (int i = 0; i < 50; i++) num[i] = i; num[5] = 10; num[55] = 100; What is the value at index 10 of the array above?
10
What does a contain after the following loop runs? int[] a = {1, 3, 7, 0, 0, 0}; int size = 3, capacity = 6, value = 5; int pos = 0; for (pos = 0; pos < size; pos++) if (a[pos] < value) break; for (int j = size; j > pos; j--) a[j] = a[j - 1]; a[pos] = value; size++; 1. { 1, 3, 5, 0, 0, 0 } 2. { 5, 1, 3, 7, 0, 0 } 3. { 1, 3, 5, 7, 0, 0 } 4. { 1, 3, 7, 5, 0, 0 }
2. { 5, 1, 3, 7, 0, 0 }
Consider the partially-filled array named a. What does the following loop do? (cin is a Scanner object) int[] a = {1, 3, 7, 0, 0, 0}; int size = 3, capacity = 6; int value = cin.nextInt(); while (size < capacity && value > 0) { a[size] = value; } 1. Reads up to 3 values and places them in the array in the unused space. 2. Reads one value and places it in the remaining first unused space endlessly. 3. Reads up to 3 values and inserts them in the array in the correct position. 4. Crashes at runtime because it tries to write beyond the array.
Reads one value and places it in the remaining first unused space endlessly.
A classification hierarchy represents an organization based on _____________ and _____________.
generalization and specialization
What is the value of alpha[3] after the following code executes? int[] alpha = new int[5]; int j; alpha[0] = 5; for (j = 1; j < 5; j++) { if (j % 2 == 0) alpha[j] = alpha[j - 1] + 2; else alpha[j] = alpha[j - 1] + 3; } 1. None of these 2. 13 3. 15 4. 10
13
What does the following loop print? int[] a = {6, 1, 9, 5, 12, 3}; int len = a.length; int x = a[0]; for (int i = 1; i < len; i++) if (a[i] < x) x = a[i]; System.out.println(x); 1. 36 2. 12 3. 1 4. 6
1
The arguments in a method call are often referred to as ____. 1. concept parameters 2. constants 3. actual parameters 4. argument lists
3. Actual parameters
When you write an explicit ____ for a class, you no longer receive the automatically written version.
constructor
What does the following loop do? int[] a = {6, 1, 9, 5, 12, 3}; int len = a.length; int x = 0; for (int i = 0; i < len; i++) if (a[i] % 2 == 0) x++; System.out.println(x); 1. Sums the even elements in a. 2. Finds the largest value in a. 3. Counts the even elements in a. 4. Finds the smallest value in a.
Counts the even elements in a.
What does the following loop do? int[] a = {6, 1, 9, 5, 12, 3}; int len = a.length; int x = 0; for (int i = 0; i < len; i++) if (a[i] % 2 == 0) x++; System.out.println(x); 1. Sums the even elements in a. 2. Counts the even elements in a. 3. Finds the smallest value in a. 4. Finds the largest value in a.
Counts the even elements in a.
Assume that you have an ArrayList variable named a containing 4 elements, and an object named element that is the correct type to be stored in the ArrayList. Which of these statements replaces the last object in the collection with element?
a.set(3, element);
Mutator methods typically begin with the word "____".
set
When you override a method defined in a superclass, you must do all of these except:
Change either the number, type, or order of parameters in the subclass.
What does the following loop print? int[] a = {6, 1, 9, 5, 12, 7}; int len = a.length; int x = 0; for (int i = 0; i < len; i++) if (a[i] % 2 == 0) x++; System.out.println(x); 1. 4 2. 22 3. 18 4. 2
2
To determine the number of valid elements in an ArrayList, use:
the size() method
A ____________ relationship exists between two classes when one class contains fields that are instances of the other class.
has-A
Examine the following UML diagram: Person -name: String +setName(String name): void +getName(): String ^ I Student -studentID: long +Student(String sname, long sid) +getID(): long 1. toString() 2. None of them 3. getName(), setName(), name 4. name, getName(), setName(), getID() 5. getName(), setName(), studentID, getID() 6. studentID, name, getName(), setName(), getID()
toString()
Consider the following statements.
public class Rectangle
{
private double length;
private double width;
public Rectangle()
{
length = 0;
width = 0;
}
public Rectangle(double l, double w)
{
length = l;
width = w;
}
public void set(double l, double w)
{
length = l;
width = w;
}
public void print()
{
System.out.println("Length = " + length
+ "; Width = " + width + "
" +
+ " Area = " + area()
+ "; Perimeter = " + perimeter());
}
public double area()
{
return length * width;
}
public void perimeter()
{
return 2 length + 2 width;
}
}
What is the output of the following statements?
3. Length = 14.0; Width = 10.0 Area = 140.0; Perimeter = 48.0
You cannot declare the same ____ name more than once within a block, even if a block contains other blocks.
variable
Which of the following lines of code implicitly calls the toString() method, assuming that pete is an initialized Student object variable?
String s = "President: " + pete;
Which of the following creates an array of 25 components of the type int? (i) int[] alpha = new[25]; (ii) int[] alpha = new int[25]; 1. Only (ii) 2. None of these 3. Only (i) 4. Both (i) and (ii)
Only (ii)
int[] num = new int[100]; for (int i = 0; i < 50; i++) num[i] = i; num[5] = 10; num[55] = 100; What is the value of num.length in the array above? 1. 0 2. 100 3. 99 4. 101
100
Examine the following UML diagram: Person -name: String +setName(String name): void +getName(): String ^ I Student -studentID: long +Student(String sname, long sid) +getID(): long Assume that pete is a Person object variable that refers to a Person object. Which method is used to print this output: pete is Person@addbf1 1. getClass() 2. getAddress() 3. getName(), getID() 4. getID() 5. getName() 6. None of them 7. toString()
toString()
Assume that you have an ArrayList variable named a containing 4 elements, and an object named element that is the correct type to be stored in the ArrayList. Which of these statements assigns the first object in the collection to the variable element?
element = a.get(0);
When you overload a method defined in a superclass, you must do which one of these? 1. Change either the number, type, or order of parameters in the subclass. 2. Use an access specifier that is at least as permissive as the superclass method 3. Return exactly the same type as the original method. 4. Use exactly the same number and type of parameters as the original method.
Change either the number, type, or order of parameters in the subclass.
public class Illustrate { private int x; private int y; public Illustrate() { x = 1; y = 2; } public Illustrate(int a) { x = a; } public void print() { System.out.println("x = " + x + ", y = " + y); } public void incrementY() { y++; } } What does the default constructor do in the class definition above? 1. Sets the value of x to 0 2. There is no default constructor. 3. Sets the value of x to 1 4. Sets the value of x to a
3. Sets the value of x to 1.
Consider the partially-filled array named a. What does the following loop do? (cin is a Scanner object) int[] a = {1, 3, 7, 0, 0, 0}; int size = 3, capacity = 6; int value = cin.nextInt(); while (value > 0) { a[size] = value; size++; value = cin.nextInt(); } 1. May crashe at runtime because it can input more elements than the array can hold 2. Reads up to 3 values and inserts them in the array in the correct position. 3. Reads one value and places it in the remaining first unused space endlessly. 4. Reads up to 3 values and places them in the array in the unused space.
1. May crashe at runtime because it can input more elements than the array can hold
Inheritance gives your programs the ability to express _______________ between classes. 1. encapsulation 2. relationships 3. composition 4. dependencies
relationships
What is stored in alpha after the following code executes? int[] alpha = new int[5]; int j; for (j = 0; j < 5; j++) { alpha[j] = j + 1; if (j > 2) alpha[j - 1] = alpha[j] + 2; } 1. alpha = {1, 2, 3, 4, 5} 2. alpha = {4, 5, 6, 7, 9} 3. alpha = {1, 5, 6, 7, 5} 4. None of these
None of these
Which of the following is used to allocate memory for the instance variables of an object of a class? 1. the reserved word public 2. the operator new 3. the operator + 4. the reserved word static
2. The operator new
What does the following loop do? int[] a = {6, 1, 9, 5, 12, 3}; int len = a.length; int x = 0; for (int i = 1; i < len; i++) if (a[i] > a[x]) x = i; System.out.println(x); 1. Finds the position of the largest value in a. 2. Sums the elements in a. 3. Finds the position of the smallest value in a. 4. Counts the elements in a.
Finds the position of the largest value in a.
If a class's only constructor requires an argument, you must provide an argument for every ____ of the class that you create. 1. object 2. type 3. parameter 4. method
1. Object
You are ____ required to write a constructor method for a class.
never
What does the following loop print? int[] a = {6, 1, 9, 5, 12, 7}; int len = a.length; int x = 0; for (int i = 0; i < len; i++) if (a[i] % 2 == 1) x++; System.out.println(x); 1. 18 2. 2 3. 22 4. 4
4
When you define a subclass, like Student, and don't supply a constructor, then its superclass must: 1. have no constructors defined 2. have no constructors defined or have an explicit default (no-arg) constructor defined. 3. have an explicit copy constructor defined 4. have an explicit default (no-arg) constructor defined 5. have an explicit working constructor defined
have no constructors defined or have an explicit default (no-arg) constructor defined.
An object is a(n) ____ of a class. 1. instance 2. method 3. field 4. constant
1. Instance
double[][] vals = {{1.1, 1.3, 1.5}, {3.1, 3.3, 3.5}, {5.1, 5.3, 5.5}, {7.1, 7.3, 7.5}} What is in vals[2][1]? 1. 1.3 2. 3.3 3. 3.5 4. 5.3
5.3
The ____________ relationship occurs when members of one class form a subset of another class, like the Animal, Mammal, Rodent example we discussed in the online lessons. 1. encapsulation 2. composition 3. is-A 4. has-A
is-A
What does a contain after the following loop? int[] a = {1, 3, 7, 0, 0, 0}; int size = 3, capacity = 6, pos = 0; for (int i = pos; i < size - 1; i++) a[i + 1] = a[i]; size--;
{1, 1, 1, 0, 0, 0}
To prevent subclasses from overriding an inherited method, the superclass can mark the method as:
final
The reference to an object that is passed to any object's nonstatic class method is called the ____. 1. this reference 2. magic number 3. literal constant 4. reference
1. This
What does a contain after the following loop? int[] a = {1, 3, 7, 0, 0, 0}; int size = 3, capacity = 6, pos = 0; for (int i = pos; i < size - 1; i++) a[i] = a[i + 1]; size--;
{3, 7, 7, 0, 0, 0}
Examine the following UML diagram: Person -name: String +setName(String name): void +getName(): String ^ I Student -studentID: long +Student(String sname, long sid) +getID(): long If you define a subclass of Person, like Student, and add a single constructor with this signature Student(String name, long id), that contains no code whatsoever, your program will only compile if the superclass (Person) contains: 1. an explicit constructor of whatever type 2. a setName() method to initialize the name field 3. an implicit constructor along with a working constructor 4. a default or no-arg constructor
a default or no-arg constructor
To use an ArrayList in your program, you must import: 1. the java.collections package 2. the java.awt package 3. Nothing; the class is automatically available, just like regular arrays. 4. the java.util package 5. the java.lang package
the java.util package
Methods that require you to use an object to call them are called ____ methods. 1. accessor 2. instance 3. internal 4. static
2. Instance
Section 33
(50 cards)
What does the following loop print? int[] a = {6, 1, 9, 5, 12, 3}; int len = a.length; int x = a[0]; for (int i = 1; i < len; i++) if (a[i] < x) x = a[i]; System.out.println(x); 1. 1 2. 12 3. 6 4. 36
1. 1
double[] as = new double[7]; double[] bs; bs = as; How many objects are present after the code fragment above is executed? 1. 2 2. 14 3. 1 4. 7
1
What does a contain after the following loop? int[] a = {1, 3, 7, 0, 0, 0}; int size = 3, capacity = 6, pos = 0; for (int i = size; i > pos; i--) a[i - 1] = a[i]; size--; 1. {3, 7, 0, 0, 0, 0} 2. {0, 0, 0, 0, 0, 0} 3. {3, 7, 7, 0, 0, 0} 4. {1, 1, 1, 0, 0, 0}
{0, 0, 0, 0, 0, 0}
Suppose you have the following declaration. int[] beta = new int[50]; Which of the following is a valid element of beta. (i) beta[0] (ii) beta[50]
Only (i)
Consider the partially-filled array named a. What does the following loop do? (cin is a Scanner object) int[] a = {1, 3, 7, 0, 0, 0}; int size = 3, capacity = 6; int value = cin.nextInt(); while (size < capacity && value > 0) { a[size] = value; size++; value = cin.nextInt(); } 1. Reads one value and places it in the remaining first unused space endlessly. 2. Crashes at runtime because it tries to write beyond the array. 3. Reads up to 3 values and places them in the array in the unused space. 4. Reads up to 3 values and inserts them in the array in the correct position.
Reads up to 3 values and places them in the array in the unused space.
Consider the following declaration. int[] alpha = new int[3]; Which of the following input statements correctly input values into alpha? (i) alpha = cin.nextInt(); alpha = cin.nextInt(); alpha = cin.nextInt(); (ii) alpha[0] = cin.nextInt(); alpha[1] = cin.nextInt(); alpha[2] = cin.nextInt(); 1. Only (i) 2. Only (ii) 3. None of these 4. Both (i) and (ii)
Only (ii)
Here is a loop that should average the contents of the array named ar : double sum = 0.0; int n = 0; for (int i=0; i < ar.length; i++, n++) sum = sum + ar[i]; if ( ?????? ) System.out.println("Average = " + ( sum / n ); What goes where the ???? appears : A. sum > 0 B. n == 0 C. n > 0 D. None of these
C. n > 0
What is stored in alpha after the following code executes? int[] alpha = new int[5]; int j; for (j = 0; j < 5; j++) { alpha[j] = j + 5; if (j % 2 == 1) alpha[j - 1] = alpha[j] + 2; }
alpha = {8, 6, 10, 8, 9}
What is stored in alpha after the following code executes? int[] alpha = new int[5]; int j; for (j = 0; j < 5; j++) { alpha[j] = j + 1; if (j > 2) alpha[j - 1] = alpha[j] + 2; }
None of these
Consider the following method which takes any number of parameters. Which statement would return the first of the numbers passed when calling the method? int first = firstNum(7, 9, 15); public int firstNum(int..nums) { // what goes here? } 1. return nums[0] 2. return nums; 3. return nums...0 4. return nums.first;
3. return nums..0
char[][] array1 = new char[15][10]; What is the value of array1.length? 1. 0 2. 10 3. 2 4. 15
4. 15
int[] hit = new hit[5]; hit[0] = 3; hit[1] = 5; hit[2] = 2; hit[3] = 6; hit[4] = 1; System.out.println(hit[1 + 3]); What is the output of the code fragment above? 1. 1 2. 5 3. 6 4. 3
1
If you pass the first element of the array ar to the method x() like this, x(ar[0]); the element ar[0] :
cannot be changed by the method x()
Which of the following statements creates alpha, an array of 5 components of the type int, and initializes each component to 10? (i) int[] alpha = {10, 10, 10, 10, 10}; (ii) int[5] alpha = {10, 10, 10, 10, 10}
Only (i)
Suppose you have the following declaration. char[] nameList = new char[100]; Which of the following range is valid for the index of the array nameList. (i) 1 through 100 (ii) 0 through 100 1. Both are invalid 2. Only (i) 3. None of these 4. Only (ii)
1. Both are invalid
What does the following loop do? int[] a = {6, 1, 9, 5, 12, 3}; int len = a.length; int x = 0; for (int i = 1; i < len; i++) if (a[i] > a[x]) x = i; System.out.println(x); 1. Finds the position of the smallest value in a. 2. Sums the elements in a. 3. Counts the elements in a. 4. Finds the position of the largest value in a.
4. Finds the position of the largest value in a.
char[][] array1 = new char[15][10]; What is the value of array1[3].length? 1. 15 2. 10 3. 2 4. 0
2. 10
What is stored in alpha after the following code executes? int[] alpha = new int[5]; int j; for (j = 0; j < 5; j++) { alpha[j] = j + 1; if (j > 2) alpha[j - 1] = alpha[j] + 2; } 1. None of these 2. alpha = {1, 2, 3, 4, 5} 3. alpha = {1, 5, 6, 7, 5} 4. alpha = {4, 5, 6, 7, 9}
1. None of these
What is the value of alpha[3] after the following code executes? int[] alpha = new int[5]; int j; for (j = 4; j >= 0; j--) { alpha[j] = j + 5; if (j <= 2) alpha[j + 1] = alpha[j] + 3; } 1. 9 2. 5 3. 8 4. 10
4. 10
What is stored in alpha after the following code executes? int[] alpha = new int[5]; int j; for (j = 0; j < 5; j++) { alpha[j] = 2 * j; if (j % 2 == 1) alpha[j - 1] = alpha[j] + j; } 1. alpha = {0, 2, 4, 6, 8} 2. alpha = {0, 2, 9, 6, 8} 3. alpha = {3, 2, 9, 6, 8} 4. alpha = {0, 3, 4, 7, 8}
alpha = {3, 2, 9, 6, 8}
What does the following loop do? int[] a = {6, 1, 9, 5, 12, 3}; int x = 0; for (int e : a) x++; 1. Sums all the elements in a. 2. Will not compile (syntax error) 3. Counts each element in a. 4. Finds the largest value in e.
3. Counts each element in a.
The first element in the array created by the statement int[] ar = { 1, 2, 3 }; is :
ar[0]
double[][] vals = {{1.1, 1.3, 1.5}, {3.1, 3.3, 3.5}, {5.1, 5.3, 5.5}, {7.1, 7.3, 7.5}} What is the value of vals[4][1] in the array above? 1. 7.1 2. There is no such value. 3. 7.3 4. 1.1
There is no such value.
To create the array variable named ar, you would use : A. int ar; B. int() ar; C. int[] ar; D. None of these
C. int[] ar;
Consider the following declaration. double[] sales = new double[50]; int j; Which of the following correctly initializes all the components of the array sales to 10. (i) for (j = 0; j < 49; j++) sales[j] = 10; (ii) for (j = 1; j <= 50; j++) sales[j] = 10;
None of these
Assume that two arrays are parallel. Assume the first array contains a person's id number and the second contains his or her age. In which index of the second array would you find the age of the person in the third index of the first array? 1. 2 2. 3 3. 1 4. It cannot be determined from the information given.
2. 3
The last element in the array created by the statement int[] ar = { 1, 2, 3 }; is :
ar[2]
What is the value of alpha[3] after the following code executes? int[] alpha = new int[5]; int j; for (j = 4; j >= 0; j--) { alpha[j] = j + 5; if (j <= 2) alpha[j + 1] = alpha[j] + 3; } 1. 8 2. 10 3. 9 4. 5
2. 10
What does a contain after the following loop runs? int[] a = {1, 3, 7, 0, 0, 0}; int size = 3, capacity = 6, value = 5; int pos = 0; for (pos = 0; pos < size; pos++) if (a[pos] > value) break; for (int j = size; j > pos; j--) a[j] = a[j - 1]; a[pos] = value; size++; 1. { 1, 3, 5, 7, 0, 0 } 2. { 1, 3, 5, 0, 0, 0 } 3. { 1, 3, 7, 5, 0, 0 } 4. { 5, 1, 3, 7, 0, 0 }
{ 1, 3, 5, 7, 0, 0 }
int[] num = new int[100]; for (int i = 0; i < 50; i++) num[i] = i; num[5] = 10; num[55] = 100; What is the index number of the last component in the array above? 1. 99 2. 100 3. 0 4. 101
1. 99
If you pass the array ar to the method m() like this, m(ar); the element ar[0] : A. will be changed by the method m() B. cannot be changed by the method m() C. may be changed by the method m(), but not necessarily D. None of these
C. may be changed by the method m(), but not necessarily
What is stored in alpha after the following code executes? int[] alpha = new int[5]; int j; for (j = 0; j < 5; j++) { alpha[j] = 2 * j; if (j % 2 == 1) alpha[j - 1] = alpha[j] + j; } 1. alpha = {0, 2, 4, 6, 8} 2. alpha = {0, 3, 4, 7, 8} 3. alpha = {0, 2, 9, 6, 8} 4. alpha = {3, 2, 9, 6, 8}
4. alpha = {3, 2, 9, 6, 8}
You can create an array of three integers by writing :
int ar[] = new int[3];
Which of the following about Java arrays is true? (i) Array components must be of the type double. (ii) The array index must evaluate to an integer.
Only (ii)
Which of the following about Java arrays is true? (i) All components must be of the same type. (ii) The array index and array element must be of the same type.
Only (i)
Which of the following statements creates alpha, a two-dimensional array of 10 rows and 5 columns, wherein each component is of the type int? 1. Both (i) and (ii) 2. Only (ii) 3. None of these 4. Only (i)
Only (ii)
What is the output of the following Java code? int[] alpha = {2, 4, 6, 8, 10}; int j; for (j = 4; j >= 0; j--) System.out.print(alpha[j] + " "); System.out.println();
10 8 6 4 2
What does the following loop print? int[] a = {6, 1, 9, 5, 12, 3}; int len = a.length; int x = a[0]; for (int i = 1; i < len; i++) if (a[i] > x) x = a[i]; System.out.println(x); 1. 36 2. 6 3. 12 4. 1
3. 12
Consider the partially-filled array named a. What does the following loop do? (cin is a Scanner object) int[] a = {1, 3, 7, 0, 0, 0}; int size = 3, capacity = 6; int value = cin.nextInt(); while (size < capacity && value > 0) { a[size] = value; size++; 1. Reads one value and places it in the remaining three unused spaces in a. 2. Reads up to 3 values and places them in the array in the unused space. 3. Reads up to 3 values and inserts them in the array in the correct position. 4. Crashes at runtime because it tries to write beyond the array.
1. Reads one value and places it in the remaining three unused spaces in a.
To print the number of elements in the array named ar, you can write : A.System.out.println(length); B.System.out.println(ar.length()); C.System.out.println(ar.length); D.System.out.println(ar.length-1);
C. System.out.println(ar.length);
Suppose we want to store a tic-tac-toe board in a program; it's a three-by-three array. It will store "O", "X" or a space at each location (that is, every location is used), and you use a two-dimensional array called board to store the individual tiles. Suppose you pass board to a method that accepts a two-dimensional array as a parameter. If the method was designed to process two-dimensional arrays of various sizes. What information about the size of the array would have to be passed to the method so it could properly process the array? A. the number of columns in the array B. the number of rows in the array C. the number of cells in the array D. Both A and B E. No additional information is needed, as the array carries its size information with it.
E. No additional information is needed, as the array carries its size information with it.
Given the method heading public void strange(int a, int b) and the declaration int[] alpha = new int[10]; int[] beta = new int[10];
strange(alpha[0], alpha[1]);
What is the output of the following Java code? int[] list = {0, 5, 10, 15, 20}; int j; for (j = 1; j <= 5; j++) System.out.print(list[j] + " "); System.out.println(); 1. 5 10 15 20 0 2. 5 10 15 20 20 3. 0 5 10 15 20 4. Code contains index out-of-bound
Code contains index out-of-bound
Consider the following declaration. Scanner cin = new Scanner(System.in); int[] beta = new int[3]; int j; Which of the following input statements correctly input values into beta? (i) beta[0] = cin.nextInt(); beta[1] = cin.nextInt(); beta[2] = cin.nextInt(); (ii) for (j = 0; j < 3; j++) beta[j] = cin.nextInt();
Both (i) and (ii)
public class ScopeRules //Line 1 { static final double rate = 10.50; static int z; static double t; public static void main(String[]args) //Line 7 { int num; double x, z; char ch; // main block... } public static void one(int f, char g) //Line 15 { // block one... } public static int w; //Line 20 public static void two(int one, int i) //Line 22 { char ch; int a; //Line 25 //block three { int x = 12; //Line 29 //... }//end block three // block two... //Line 32 } } Which of the following identifiers is visible in method one? 1. x (block three's local variable) 2. local variables of method two 3. rate (before main) 4. one (method two's formal parameter)
rate (before main)
Which of the following creates an array of 25 components of the type int? (i) int[] alpha = new[25]; (ii) int[] alpha = new int[25];
Only (ii)
Local variables A. Lose the values stored in them between calls to the method in which the variable is declared B. May have the same name as local variables in other methods C. Are hidden from other methods D. All of the above
All of the above
public class ScopeRules //Line 1 { static final double rate = 10.50; static int z; static double t; public static void main(String[]args) //Line 7 { int num; double x, z; char ch; // main block... } public static void one(int f, char g) //Line 15 { // block one... } public static int w; //Line 20 public static void two(int one, int i) //Line 22 { char ch; int a; //Line 25 //block three { int x = 12; //Line 29 //... }//end block three // block two... //Line 32 } } Which of the following identifiers is visible on the line marked // main block? 1. All identifiers are visible in main. 2. local variables of method two 3. w (before method two) 4. z (before main)
w (before method two)
To print the last element in the array named ar, you can write : A. System.out.println(ar.length); B. System.out.println(ar[length]); C. System.out.println(ar[ar.length]); D. None of these
D
What is the value of alpha[4] after the following code executes? int[] alpha = new int[5]; int j; alpha[0] = 2; for (j = 1; j < 5; j++) alpha[j] = alpha[j - 1] + 3; 1. 11 2. 14 3. 5 4. 8
2. 14
Section 34
(50 cards)
The header of a value-returning method must specify this. A. The method's local variable name B. The data type of the return value C. The name of the variable in the calling program that will receive the returned value D. All of the above
The data type of the return value
Which of the following would be a valid method call for the following method? public static void showProduct (int num1, double num2) { int product; product = num1 * (int)num2; System.out.println("The product is " + product); } A. showProduct(10.0, 4); B. showProduct(33.0, 55.0); C. showProduct(5.5, 4.0); D. showProduct(10, 4.5);
D. showProduct(10, 4.5);
When a method tests an argument and returns a true or false value, it should return
A boolean value
public class ScopeRules //Line 1 { static final double rate = 10.50; static int z; static double t; public static void main(String[]args) //Line 7 { int num; double x, z; char ch; // main block... } public static void one(int f, char g) //Line 15 { // block one... } public static int w; //Line 20 public static void two(int one, int i) //Line 22 { char ch; int a; //Line 25 //block three { int x = 12; //Line 29 //... }//end block three // block two... //Line 32 } } Which of the following is an example of a local identifier in the example above? 1. w (line 20) 2. t (line 5) 3. a (line 25) 4. rate (line 3)
3. a (line 25)
Which of the following is not a benefit derived from using methods in programming? A. Code reuse B. Problems are more easily solved C. Simplifies programs D. All of the above are benefits
D. All of the above are benefits
In a @return tag statement the description A. Describes the return value B. Must be longer than one line C. Cannot be longer than one line D. Describes the parameter values
Describes the return value
public class ScopeRules //Line 1 { static final double rate = 10.50; static int z; static double t; public static void main(String[]args) //Line 7 { int num; double x, z; char ch; // main block... } public static void one(int f, char g) //Line 15 { // block one... } public static int w; //Line 20 public static void two(int one, int i) //Line 22 { char ch; int a; //Line 25 //block three { int x = 12; //Line 29 //... }//end block three // block two... //Line 32 } } Where is identifier x (block three's local variable) visible? 1. In two and block three 2. In block three and main 3. In block three only 4. In one and block three
In block three only
methods are commonly used to
Break a problem down into small manageable pieces.
public static int minimum(int x, int y) { int smaller; if (x < y) smaller = x; else smaller = y; return smaller; } What is the name of the method above?
minimum
What will be returned from the following method? public static double methodA() { double a = 8.5 + 9.5; return a; }
18.0
When an object, such as a String, is passed as an argument, it is
Actually a reference to the object that is passed
Which of the following is not part of a method call? A. Return type B. Parentheses C. Method name D. All of the above are part of a method call
Return type
All @param tags in a method's documentation comment must
Appear after the general description of the method
public static int minimum(int x, int y) { int smaller; if (x < y) smaller = x; else smaller = y; return smaller; } Based on the above code, what would be the output of the statement int s = minimum(5, minimum(3, 7));
3
When an argument is passed to a method,
Its value is copied into the method's parameter variable
What is wrong with the following method call? displayValue (double x);
The data type of the argument x should not be included in the method call
public class ScopeRules //Line 1 { static final double rate = 10.50; static int z; static double t; public static void main(String[]args) //Line 7 { int num; double x, z; char ch; // main block... } public static void one(int f, char g) //Line 15 { // block one... } public static int w; //Line 20 public static void two(int one, int i) //Line 22 { char ch; int a; //Line 25 //block three { int x = 12; //Line 29 //... }//end block three // block two... //Line 32 } } Which of the following identifiers is visible in block three? x (one's formal parameter) t (before main) z (before main) local variables of main
t (before main)
What will be returned from the following method? public static double methodA() { double a = 8.5 + 9.5; return a; }
18.0
public class ScopeRules //Line 1 { static final double rate = 10.50; static int z; static double t; public static void main(String[]args) //Line 7 { int num; double x, z; char ch; // main block... } public static void one(int f, char g) //Line 15 { // block one... } public static int w; //Line 20 public static void two(int one, int i) //Line 22 { char ch; int a; //Line 25 //block three { int x = 12; //Line 29 //... }//end block three // block two... //Line 32 } } Which of the following identifiers is NOT visible in block three? 1. t (before main) 2. z (before main) 3. local variables of method two 4. main
z (before main)
public static String exampleMethod(int n, char ch) Which of the following statements about the method heading above is NOT true? 1. The method has two parameters. 2. exampleMethod is an identifier giving a name to this specific method. 3. The method cannot be used outside the class. 4. It is a value-returning method of type String.
3. The method cannot be used outside the class.
Which of the following is NOT true about return statements? 1. A method can have more than one return statement. 2. return statements can be used in void methods to return values. 3. Whenever a return statement executes in a method, the remaining statements are skipped and the method exits. 4. A value-returning method returns its value via the return statement.
2. return statements can be used in void methods to return values.
public static String exampleMethod(int n, char ch) Based on the method heading above, what is the return type of the value returned? 1. char 2. String 3. int 4. Nothing will be returned
String
What is wrong with the following method call? displayValue (double x);
The data type of the argument x should not be included in the method call
Methods are commonly used to
Break a problem down into small manageable pieces
This part of a method is a collection of statements that are performed when the method is executed.
Method body
Which modifier is used to specify that a method cannot be used outside a class?
private
Formal parameters of primitive data types provide ____ between actual parameters and formal parameters.
a one-way link
public static int minimum(int x, int y) { int smaller; if (x < y) smaller = x; else smaller = y; return smaller; } Based on the above code, what would be the output of the statement int s = minimum(5, minimum(3, 7)); 1. 5 2. There would be no output; this is not a valid statement. 3. 7 4. 3
3
When an object, such as a String, is passed as an argument, it is A. Passed by value like any other parameter value B. Actually a reference to the object that is passed C. Encrypted D. Necessary to know exactly how long the string is when writing the program
Actually a reference to the object that is passed
public static int minimum(int x, int y) { int smaller; if (x < y) smaller = x; else smaller = y; return smaller; } What is the return type of the method above?
int
Given the following method header, which of the method calls would cause an error? public void displayValues(int x, int y)
displayValue(a,b); // where a is a short and b is a long
public static int minimum(int x, int y) { int smaller; if (x < y) smaller = x; else smaller = y; return smaller; } Which of the following is a valid call to the method above? 1. minimum(int x, int y); 2. minimum(int 5, int 4); 3. minimum(5, 4); 4. public static int minimum(5, 4);
minimum(5, 4);
In a @return tag statement the description
Describes the return value
This type of method method performs a task and sends a value back to the code that called it. A. Local B. Void C. Complex D. Value-returning
Value-returning
public static int minimum(int x, int y) { int smaller; if (x < y) smaller = x; else smaller = y; return smaller; } Which of the following is a valid call to the method above?
minimum(5, 4);
Which of the following is NOT a modifier?
return
Which of the following is NOT an actual parameter? 1. Expressions used in a method call 2. Constant values used in a method call 3. Variables used in a method call 4. Variables defined in a method heading
Variables defined in a method heading
Functional decomposition is
The process of breaking a problem down into smaller pieces.
public static int minimum(int x, int y) { int smaller; if (x < y) smaller = x; else smaller = y; return smaller; } Which of the following is a valid call to the method above?
minimum(5, 4);
public class ScopeRules //Line 1 { static final double rate = 10.50; static int z; static double t; public static void main(String[]args) //Line 7 { int num; double x, z; char ch; // main block... } public static void one(int f, char g) //Line 15 { // block one... } public static int w; //Line 20 public static void two(int one, int i) //Line 22 { char ch; int a; //Line 25 //block three { int x = 12; //Line 29 //... }//end block three // block two... //Line 32 } } Which of the following identifiers is NOT visible in method two?
x (variable in block three)
Which of the following statements is NOT true?
The operator new must always be used to allocate space of a specific type, regardless of the type.
Which of the following statements is NOT true? 1. Reference variables contain the address of the memory space where the data is stored. 2. The operator new must always be used to allocate space of a specific type, regardless of the type. 3. Reference variables do not directly contain the data. 4. A String variable is actually a reference variable of the type String.
2. The operator new must always be used to allocate space of a specific type, regardless of the type.
public static int minimum(int x, int y) { int smaller; if (x < y) smaller = x; else smaller = y; return smaller; } Which of the following is NOT part of the heading of the method above? 1. static 2. int smaller; 3. minimum(int x, int y) 4. public
2. int smaller;
In the following code, System.out.println(num), is an example of ________. double num = 5.4; System.out.println(num); num = 0.0;
A void method
What is an actual parameter?
The value passed into a method by a method call
If method A calls Method B, and method B calls method C, and method C calls method D, when method D finishes, what happens?
Control is returned to method C
The header of a value-returning method must specify this.
The data type of the return value
The lifetime of a method's local variable is
Only while the method is executing
In the following code, System.out.println(num), is an example of ________. double num = 5.4; System.out.println(num); num = 0.0;
A void method
This type of method method performs a task and sends a value back to the code that called it.
Value-returning
Section 35
(50 cards)
Consider the following method definition. public int strange(int[] list, int item) { int count; for (int j = 0; j < list.length; j++) if (list[j] == item) count++; return count; } Which of the following statements best describe the behavior of this method? 1. This method returns the sum of all the values of list. 2. None of these 3. This method returns the number of times item is stored in list. 4. This method returns the number of values stored in list.
3. This method returns the number of times item is stored in list.
If you pass the first element of the array ar to the method x() like this, x(ar[0]); the element ar[0] : A. will be changed by the method x() B. cannot be changed by the method x() C. may be changed by the method x(), but not necessarily D. None of these
B. cannot be changed by the method x()
The last element in the array created by the statement int[] ar = { 1, 2, 3 }; is : A. ar[3] B. ar[2] C. ar(3) D. ar(2)
B. ar[2]
double[][] vals = {{1.1, 1.3, 1.5}, {3.1, 3.3, 3.5}, {5.1, 5.3, 5.5}, {7.1, 7.3, 7.5}} How many rows are in the array above? 1. 4 2. 0 3. 2 4. 3
1. 4
Consider the following statements. public class PersonalInfo { private String name; private int age; private double height; private double weight; public void set(String s, int a, double h, double w) { name = s; age = a; height = h; weight = w; } public String getName() { return name; } public int getAge() { return age; } public double getHeight() { return height; } public double getWeight() { return weight; } } Assume you have the following code fragment inside a method in a different class: PersonalInfo person1 = new PersonalInfo(); PersonalInfo person2 = new PersonalInfo(); String n; int a; double h, w; Which of the following statements are valid (both syntactically and semantically) in Java? (i) person2 = person1; (ii) n = person1.getName(); a = person1.getAge(); h = person1.getHeight(); w = person1.getWeight(); person2.set(n, a, h, w); 1. None of these 2. Only (ii) 3. Only (i) 4. Both (i) and (ii)
4. Both (i) and (ii)
Assigning ____ to a field means that no other classes can access the field's values. 1. key access 2. user rights 3. private access 4. protected access
3. private access
The keyword ____ indicates that a field value is non-changable. 1. const 2. readonly 3. final 4. static
3. final
Suppose you have the following declaration. double[] salesData = new double[1000]; Which of the following range is valid for the index of the array salesData. (i) 0 through 999 (ii) 1 through 1000 1. Only (i) 2. None of these 3. Both are invalid 4. Only (ii)
1. Only (i)
char[][] array1 = new char[15][10]; How many dimensions are in the array above? 1. 3 2. 2 3. 1 4. 0
2. 2
Here is a loop that is supposed to sum each of the values in the array named ar : double sum = 0.0; for (int i=0; i < ar.length; i++) // ??? System.out.println(sum); What goes on the line containing ???? : A. i = sum + ar[i]; B. sum + ar[i]; C. sum = sum + i; D. sum += ar[i];
D. sum += ar[i];
Which of the following is the array subscripting operator in Java? 1. . 2. {} 3. [] 4. new
3. []
What is the value of alpha[3] after the following code executes? int[] alpha = new int[5]; int j; alpha[0] = 5; for (j = 1; j < 5; j++) { if (j % 2 == 0) alpha[j] = alpha[j - 1] + 2; else alpha[j] = alpha[j - 1] + 3; } 1. 13 2. 15 3. None of these 4. 10
1. 13
To print the last element in the array named ar, you can write : A. System.out.println(ar.length); B. System.out.println(ar[length]); C. System.out.println(ar[ar.length]); D. None of these
D. None of these
int[] hits = {10, 15, 20, 25, 30}; copy(hits); What is being passed into copy in the method call above? 1. 10 2. A reference to the array object hits 3. The value of the elements of hits 4. A copy of the array hits
2. A reference to the array object hits
The value of the last element in the array created by the statement int[] ar = new int[3]; is : A. 0 B. 1 C. 2 D. 3
A. 0
What does a contain after the following loop? int[] a = {1, 3, 7, 0, 0, 0}; int size = 3, capacity = 6, pos = 0; for (int i = size; i > pos; i--) a[i - 1] = a[i]; size--; 1. {3, 7, 7, 0, 0, 0} 2. {1, 1, 1, 0, 0, 0} 3. {3, 7, 0, 0, 0, 0} 4. {0, 0, 0, 0, 0, 0}
4. {0, 0, 0, 0, 0, 0}
What does the following loop print? int[] a = {6, 9, 1, 5, 12, 3}; int len = a.length; int x = 0; for (int i = 1; i < len; i++) if (a[i] < a[x]) x = i; System.out.println(x); 1. 6 2. 1 3. 2 4. 4
3. 2
Suppose we define a class called NameInfo. It has several private fields, including String hospName; and several public methods, including one that accesses hospName: String getName(); Now, we define an array like this: NameInfo[] listOfHospitals = new NameInfo[100]; Which expression below correctly accesses the name of the hospital that's stored in position 5 (that is, the 6th element) of the listOfHospitals? A. listOfHospitals.getName(5) B. listOfHospitals.hospName[5] C. listOfHospitals[5].hospName D. listOfHospitals[getName(5)] E. listOfHospitals[5].getName()
E. listOfHospitals[5].getName()
How can a method send a primitive value back to the caller?
By using the return statement
Which of the following class definitions is correct in Java? (i) public class Student { private String name; private double gpa; private int id; public void Student() { name = ""; gpa = 0; id = 0; } public void Student(String s, double g, int i) { set(s, g, i); } public void set(String s, double g, int i) { name = s; gpa = g; id = i; } public void print() { System.out.println(name + " " + id + " " + gpa); } } (ii) public class Student { private String name; private double gpa; private int id; public Student() { name = ""; gpa = 0; id = 0; } public Student(String s, double g, int i) { set(s, g, i); } public void set(String s, double g, int i) { name = s; gpa = g; id = i; } public void print() { System.out.println(name + " " + id + " " + gpa); } } 1. Only (i) 2. Both (i) and (ii) 3. None of these 4. Only (ii)
4. Only (ii)
You can create an array of three doubles by writing : A. double ar[] = 3; B. double ar[3]; C. double ar[] = { 1.1, 2.2, 3.3 }; D. double[] ar = double(3);
C. double ar[] = { 1.1, 2.2, 3.3 };
What does the following loop do? int[] a = {6, 1, 9, 5, 12, 7}; int len = a.length; int x = 0; for (int i = 0; i < len; i++) if (a[i] % 2 == 1) x += a[i]; System.out.println(x); 1. Finds the largest value in a. 2. Finds the smallest value in a. 3. Counts the odd elements in a. 4. Sums the odd elements in a.
4. Sums the odd elements in a.
public static int minimum(int x, int y) { int smaller; if (x < y) smaller = x; else smaller = y; return smaller; } Which of the following is a valid call to the method above?
minimum(5, 4);
What does the following loop do? int[] a = {6, 1, 9, 5, 12, 3}; int len = a.length; int x = 0; for (int i = 1; i < len; i++) if (a[i] < a[x]) x = i; System.out.println(x); 1. Finds the position of the smallest value in a. 2. Sums the elements in a. 3. Finds the position of the largest value in a. 4. Counts the elements in a.
1. Finds the position of the smallest value in a.
double[][] vals = {{1.1, 1.3, 1.5}, {3.1, 3.3, 3.5}, {5.1, 5.3, 5.5}, {7.1, 7.3, 7.5}} What is in vals[2][1]? 1. 3.3 2. 3.5 3. 1.3 4. 5.3
4. 5.3
What does the following loop do? int[] a = {6, 1, 9, 5, 12, 3}; for (int e : a) e = 0; 1. Changes each element in a to 0. 2. Compiles, but crashes when run. 3. Changes each element e to 0 but has no effect on the corresponding element in a. 4. Will not compile (syntax error)
3. Changes each element e to 0 but has no effect on the corresponding element in a.
Which of the following methods returns an array ? A. int acpy(int n) { ... } B. int acpy(int[] n) { ... } C. int[] acpy(int n) { ... } D. None of these
C. int[] acpy(int n) { ... }
What does a contain after the following loop runs? int[] a = {1, 3, 7, 0, 0, 0}; int size = 3, capacity = 6, value = 5; int pos = 0; for (pos = 0; pos < size; pos++) if (a[pos] < value) break; for (int j = size; j > pos; j--) a[j] = a[j - 1]; a[pos] = value; size++; 1. { 1, 3, 7, 5, 0, 0 } 2. { 1, 3, 5, 0, 0, 0 } 3. { 1, 3, 5, 7, 0, 0 } 4. { 5, 1, 3, 7, 0, 0 }
4. { 5, 1, 3, 7, 0, 0 }
What is the value of alpha[4] after the following code executes? int[] alpha = new int[5]; int j; alpha[0] = 2; for (j = 1; j < 5; j++) alpha[j] = alpha[j - 1] + 3; 1. 5 2. 11 3. 8 4. 14
4. 14
What is the value of alpha[2] after the following code executes? int[] alpha = new int[5]; int j; for (j = 0; j < 5; j++) alpha[j] = 2 * j + 1; 1. 1 2. 4 3. 5 4. 6
3. 5
Which of the following statements about strings is NOT true? 1. The class String contains many methods that allow you to change an existing string. 2. A String variable is actually a reference variable. 3. When a string is assigned to a String variable, the string cannot be changed. 4. When you use the assignment operator to assign a string to a String variable, the computer allocates memory space large enough to store the string.
The class String contains many methods that allow you to change an existing string.
int[] x = new int[10]; x[0] = 34; x[1] = 88; println(x[0] + " " + x[1] + " " + x[10]); What is the output of the code fragment above? 1. 34 88 88 2. This program throws an exception. 3. 34 88 0 4. 0 34 88
2. This program throws an exception.
Which of the following statements is NOT true? 1. Reference variables contain the address of the memory space where the data is stored. 2. The operator new must always be used to allocate space of a specific type, regardless of the type. 3. Reference variables do not directly contain the data. 4. A String variable is actually a reference variable of the type String.
2. The operator new must always be used to allocate space of a specific type, regardless of the type.
A(n) ____ method is a method that creates and initializes objects. 1. constructor 2. non-static 3. instance 4. accessor
1. constructor
What is the value of alpha[2] after the following code executes? int[] alpha = new int[5]; int j; for (j = 0; j < 5; j++) alpha[j] = 2 * j + 1; 1. 4 2. 6 3. 1 4. 5
4. 5
What does the following loop do? int[] a = {1, 3, 7, 0, 0, 0}; int size = 3, capacity = 6, value = 5; int pos = 0; for (pos = 0; pos < size; pos++) if (a[pos] > value) break; for (int j = size; j > pos; j--) a[j] = a[j - 1]; a[pos] = value; size++; 1. Removes (deletes) value from a so that the array remains ordered. 2. Places value in the last element of a 3. Inserts value into a so that the array remains ordered. 4. Places value in the first unused space in a (where the first 0 appears)
3. Inserts value into a so that the array remains ordered.
Consider the following statements. public class Circle { private double radius; public Circle() { radius = 0.0; } public Circle(double r) { radius = r; } public void set(double r) { radius = r; } public void print() { System.out.println(radius + " " + area + " " + circumference()); } public double area() { return 3.14 radius radius; } public double circumference() { return 2 3.14 radius; } } Assume, also that you have the following two statements appearing inside a method in another class: Circle myCircle = new Circle(); double r; Which of the following statements are valid (both syntactically and semantically) in Java? (Assume that cin is Scanner object initialized to the standard input device.) (i) r = cin.nextDouble(); myCircle.area = 3.14 r r; System.out.println(myCircle.area); (ii) r = cin.nextDouble(); myCircle.set(r); System.out.println(myCircle.area()); 1. Only (ii) 2. Both (i) and (ii) 3. Only (i) 4. None of these
1. Only (ii)
Consider the following class definition. public class Rectangle { private double length; private double width; public Rectangle() { length = 0; width = 0; } public Rectangle(double l, double w) { length = l; width = w; } public void set(double l, double w) { length = l; width = w; } public void print() { System.out.println(length + " " + width); } public double area() { return length * width; } public double perimeter() { return 2 length + 2 width; } } Suppose that you have the following declaration. Rectangle bigRect = new Rectangle(10, 4); Which of the following statements are valid in Java? (Assume that cin is Scanner object initialized to the standard input device.) (i) bigRect.length = cin.nextDouble(); bigRect.width = cin.nextDouble(); (ii) double l; double w; l = cin.nextDouble(); w = cin.nextDouble(); bigRect.set(l, w); 1. Only (i) 2. Only (ii) 3. Both (i) and (ii) 4. None of these
2. Only (ii)
int[] array1 = {1, 3, 5, 7}; for (int i = 0; i < array1.length; i++) if (array1[i] > 5) System.out.println(i + " " + array1[i]); What is the output of the code fragment above? 1. 0 3 2. 7 3 3. 5 1 4. 3 7
4. 3 7
What does the following loop do? int[] a = {6, 1, 9, 5, 12, 3}; int len = a.length; int x = 0; for (int i = 0; i < len; i++) if (a[i] % 2 == 0) x += a[i]; System.out.println(x); 1. Sums the even elements in a. 2. Finds the largest value in a. 3. Finds the smallest value in a. 4. Counts the even elements in a.
1. Sums the even elements in a.
You can use ____ arguments to initialize field values, but you can also use arguments for any other purpose. 1. constructor 2. field 3. data 4. object
1. constructor
What is stored in alpha after the following code executes? int[] alpha = new int[5]; int j; for (j = 0; j < 5; j++) { alpha[j] = j + 5; if (j % 2 == 1) alpha[j - 1] = alpha[j] + 2; } 1. alpha = {5, 6, 10, 8, 9} 2. alpha = {8, 6, 7, 8, 9} 3. alpha = {5, 6, 7, 8, 9} 4. alpha = {8, 6, 10, 8, 9}
4. alpha = {8, 6, 10, 8, 9}
A(n) ____ constructor is one that requires no arguments. Student Response Value Correct Answer Feedback 1. write 2. default 3. class 4. explicit
2. default
Which of the following words indicates an object's reference to itself? 1. public 2. that 3. this 4. protected
3. this
When an object, such as a String, is passed as an argument, it is
Actually a reference to the object that is passed
Methods are commonly used to
Break a problem down into small manageable pieces
A(n) ____ variable is known only within the boundaries of the method. 1. instance 2. double 3. local 4. method
3. local
Consider the following declaration. int[] list = new int[10]; int j; int sum; Which of the following correctly finds the sum of the elements of list? (i) sum = 0; for (j = 0; j < 10; j++) sum = sum + list[j]; (ii) sum = list[0]; for (j = 1; j < 10; j++) sum = sum + list[j]; 1. Only (ii) 2. Only (i) 3. Both (i) and (ii) 4. None of these
3. Both (i) and (ii)
int[] num = new int[100]; for (int i = 0; i < 50; i++) num[i] = i; num[5] = 10; num[55] = 100; How many components are in the array above? 1. 100 2. 0 3. 50 4. 99
1. 100
public class Illustrate { private int x; private int y; public Illustrate() { x = 1; y = 2; } public Illustrate(int a) { x = a; } public void print() { System.out.println("x = " + x + ", y = " + y); } public void incrementY() { y++; } } How many constructors are present in the class definition above? 1. 3 2. 2 3. 1 4. 0
2. 2
Section 36
(50 cards)
How many constructors can a class have? 1. 2 2. Any number 3. 1 4. 0
2. Any number
Methods that retrieve values are called ____ methods. 1. static 2. mutator 3. accessor 4. class
3. accessor
Examine the following UML diagram: Person -name: String +setName(String name): void +getName(): String ^ I Student -studentID: long +Student(String sname, long sid) +getID(): long Which of these fields or methods are inherited by the Person class? 1. getName(), setName(), studentID, getID() 2. getName(), setName(), name 3. None of them 4. studentID, name, getName(), setName(), getID() 5. name, getName(), setName(), getID()
None of them
Which of these are superclass members are not inherited by a subclass? 1. a protected method 2. a protected instance variable 3. a public instance variable 4. a public constructor 5. a public method
4. a public constructor
When you define a subclass, like Student, and add a single constructor with this signature Student(String name, long id), an implicit first line is added to your constructor. That line is: 1. No implicit lines are added to your constructor 2. this(); 3. super(name, id); 4. base(); 5. super();
5. super();
How many constructors can a class have? 1. Any number 2. 2 3. 1 4. 0
1. Any number
Mutator methods typically have a return type of ______________. 1. boolean 2. String 3. void 4. int
3. void
A locally declared variable always ____ another variable with the same name elsewhere in the class. 1. uses 2. creates 3. deletes 4. masks
4. masks
Mutator methods typically begin with the word "____". 1. read 2. get 3. call 4. set
4. set.
Which of the following lines of code explicitly calls the toString() method, assuming that pete is an initialized Student object variable? 1. println(pete.toString()); 2. println("" + pete); 3. println(super.toString()); 4. println(pete)
1. println(pete.toString());
Consider the following class definition. public class Rectangle { private double length; private double width; public Rectangle() { length = 0; width = 0; } public Rectangle(double l, double w) { length = l; width = w; } public void set(double l, double w) { length = l; width = w; } public void print() { System.out.println(length + " " + width); } public double area() { return length * width; } public void perimeter() { return 2 length + 2 width; } } Suppose that you have the following declaration. Rectangle bigRect = new Rectangle(); Which of the following sets of statements are valid in Java? (i) bigRect.set(10, 5); (ii) bigRect.length = 10; bigRect.width = 5; 1. Only (ii) 2. None of these 3. Only (i) 4. Both (i) and (ii)
3. Only (i)
Consider the following class definition. public class Rectangle { private double length; private double width; private double area; private double perimeter; public Rectangle() { length = 0; width = 0; } public Rectangle(double l, double w) { length = l; width = w; } public void set(double l, double w) { length = l; width = w; } public void print() { System.out.println(length + " " + width); } public double area() { return length * width; } public double perimeter() { return 2 length + 2 width; } } Suppose that you have the following declaration. Rectangle bigRect = new Rectangle(10, 4); Which of the following sets of statements are valid (both syntactically and semantically) in Java? (i) bigRect.area(); bigRect.perimeter(); bigRect.print(); (ii) bigRect.area = bigRect.area(); bigRect.perimeter = bigRect.perimeter(); bigRect.print(); 1. Both (i) and (ii) 2. Only (ii) 3. None of these 4. Only (i)
4. Only (i)
If a method in the superclass is declared protected, you may replace it in the subclass as long as the subclass method that you define: 1. has a different number, type, or order of parameters 2. has a different return type 3. is defined as protected or private 4. is defined as public or protected 5. is defined only as public
4. is defined as public or protected
A variable comes into existence, or ____, when you declare it. 1. is referenced 2. comes into scope 3. overrides scope 4. goes out of scope
2. comes into scope
Suppose we have these two array definitions in main(): String[] hospName = new String[100]; int[] hospCode = new int[100]; hospCode[I] stores the code for a specific hospital. hospName[J] stores the name of the hospital that has code J. Suppose the code 24 is stored at position 13 of hospCode. Which expression below references 24's corresponding name? A. hospName[13] B. hospCode[24] C. hospCode[hospName[13]] D. hospName[hospCode[13]] E. hospName[hospCode[24]]
D. hospName[hospCode[13]] General Feedback: What we want is the name for hospital with code 24. We could get that with hospName[24], but that's not one of the options. However, we know that hospCode[13] contains the value 24, so we can use that instead.
In the type-safe version of the ArrayList class, you specify the type of each element by: 1. None of these; an ArrayList can hold any object type 2. specifying a generic type parameter when defining the ArrayList variable 3. using the special type-safe version of the subscript operator 4. passing a Class parameter as the first argument to the constructor 5. using the setType() method once the ArrayList is constructed
specifying a generic type parameter when defining the ArrayList variable
To access the name instance variable from inside a method defined in the Student class, name must use the _____________ modifier. (We'll ignore the default package-private access if no modifier is used.) 1. public 2. protected or public 3. private or protected 4. private 5. protected 6. Any of them
2. protected or public
When you define a subclass, like Student, and don't supply a constructor, then its superclass must: 1. have an explicit working constructor defined 2. have no constructors defined 3. have an explicit default (no-arg) constructor defined 4. have an explicit copy constructor defined 5. have no constructors defined or have an explicit default (no-arg) constructor defined.
5. have no constructors defined or have an explicit default (no-arg) constructor defined.
Read a double from the Scanner object and store it in a double
Java: double varX= input.nextDouble();
Assume you have a Student object and a Student variable named bill that refers to your Student object. Which of these statements would be legal? bill.name = "Bill Gates"; // I bill.setName("Bill Gates"); // II println(bill.getName()); // III bill.studentID = 123L; // IV println(bill.getID()); // V 1. II, III, IV, VI 2. IV, V 3. All of them 4. II, III, V 1 5. None of them
4. II, III, V
What does the following loop do? int[] a = {6, 1, 9, 5, 12, 3}; int len = a.length; int x = 0; for (int i = 0; i < len; i++) if (a[i] % 2 == 0) x += a[i]; System.out.println(x); 1. Counts the even elements in a. 2. Sums the even elements in a. 3. Finds the largest value in a. 4. Finds the smallest value in a.
2. Sums the even elements in a.
The inside block, which is contained entirely within an outside block, is referred to as ____. 1. nested 2. out of scope 3. ambiguous 4. a reference
1. nested
Prompt the user
Java: System.out.println
Read an int from the Scanner object and store it in an int
Java: Int varx= input.nextint();
Create a Method called methodX
Java: public static void methodX(){
What does a contain after the following loop runs? int[] a = {1, 3, 7, 0, 0, 0}; int size = 3, capacity = 6, value = 5; int pos = 0; for (pos = 0; pos < size; pos++) if (a[pos] > value) break; for (int j = size; j > pos; j--) a[j] = a[j - 1]; a[pos] = value; size++; 1. { 5, 1, 3, 7, 0, 0 } 2. { 1, 3, 7, 5, 0, 0 } 3. { 1, 3, 5, 7, 0, 0 } 4. { 1, 3, 5, 0, 0, 0 }
3. { 1, 3, 5, 7, 0, 0 }
Examine the following UML diagram: Person -name: String +setName(String name): void +getName(): String ^ I Student -studentID: long +Student(String sname, long sid) +getID(): long Which of these fields or methods are inherited by the Person class? 1. toString() 2. getName(), setName(), studentID, getID() 3. studentID, name, getName(), setName(), getID() 4. name, getName(), setName(), getID() 5. getName(), setName(), name 6. None of them
toString()
Which of the following lines of code explicitly calls the toString() method, assuming that pete is an initialized Student object variable? 1. println(super.toString()); 2. println("" + pete); 3. println(pete); 4. println(pete.toString());
4. println(pete.toString());
Create a Scanner object that reads from System.in
Java: Scanner input= new Scanner(System.in)
When you create a new class using inheritance, you use the extends keyword to specify the __________________ when you define the ________________. 1. derived class, subclass 2. derived class, base class 3. superclass, base class 4. subclass, base class 5. superclass, subclass 6. subclass, superclass
5. superclass, subclass
Methods that retrieve values are called ____ methods. 1. accessor 2. class 3. mutator 4. static
1. accessor
public class ScopeRules //Line 1 { static final double rate = 10.50; static int z; static double t; public static void main(String[]args) //Line 7 { int num; double x, z; char ch; // main block... } public void one(int f, char g) //Line 15 { // block one... } public static int w; //Line 20 public void two(int one, int i) //Line 22 { char ch; int a; //Line 25 //block three { int z = 12; //Line 29 //... }//end block three // block two... //Line 32 } } Where is identifier z (block three's local variable) visible? 1. In block three and main 2. In block three only 3. In two and block three 4. In one and block three
2. In block three only
Assume that you have an ArrayList variable named a containing 4 elements, and an object named element that is the correct type to be stored in the ArrayList. Which of these statements adds the object to the end of the collection? 1. a[3] = element; 2. a[4] = element; 3. a.add(element); 4. a.add(element, 4);
3. a.add(element);
Consider the following class definition. public class Rectangle { private double length; private double width; private double area; private double perimeter; public Rectangle() { length = 0; width = 0; } public Rectangle(double l, double w) { length = l; width = w; } public void set(double l, double w) { length = l; width = w; } public void print() { System.out.println(length + " " + width); } public double area() { return length * width; } public double perimeter() { return 2 length + 2 width; } } Suppose that you have the following declaration. Rectangle bigRect = new Rectangle(10, 4); Which of the following sets of statements are valid (both syntactically and semantically) in Java? (i) bigRect.area(); bigRect.perimeter(); bigRect.print(); (ii) bigRect.area = bigRect.area(); bigRect.perimeter = bigRect.perimeter(); bigRect.print(); 1. Only (ii) 2. None of these 3. Only (i) 4. Both (i) and (ii)
Only (i)
What does the following loop do? int[] a = {6, 1, 9, 5, 12, 3}; int len = a.length; int x = 0; for (int i = 1; i < len; i++) if (a[i] > a[x]) x = i; System.out.println(x); 1. Sums the elements in a. 2. Finds the position of the smallest value in a. 3. Finds the position of the largest value in a. 4. Counts the elements in a.
3. Finds the position of the largest value in a.
Programmers frequently use the same name for a(n) ____ and an argument to a method simply because it is the "best name" to use. 1. block 2. instance field 3. variable 4. constant
2. instance field
Import the Scanner Class
Java: import java.util.Scanner;//at top of the file
public class ScopeRules //Line 1 { static final double rate = 10.50; static int z; static double t; public static void main(String[]args) //Line 7 { int num; double x, z; char ch; // main block... } public void one(int f, char g) //Line 15 { // block one... } public static int w; //Line 20 public void two(int one, int i) //Line 22 { char ch; int a; //Line 25 //block three { int z = 12; //Line 29 //... }//end block three // block two... //Line 32 } } Which of the following identifiers is NOT visible in method two? 1. x (variable in main block) 2. w (before method two) 3. rate (before main) 4. one (method name)
x (variable in main block)
Assume that you have an ArrayList variable named a containing 4 elements, and an object named element that is the correct type to be stored in the ArrayList. Which of these statements replaces the first object in the collection with element? 1. a.set(0, element); 2. a.set(element, 0); 3. a.add(0, element); 4. a[0] = element;
1. a.set(0, element);
Assume that you have an ArrayList variable named a containing 4 elements, and an object named element that is the correct type to be stored in the ArrayList. Which of these statements adds the object to the beginning of the collection? 1. a[4] = element; 2. a.add(0, element); 3. a.insert(element); 4. a.add(element, 0); 5. a[0] = element;
2. a.add(0, element);
_______________—the specification of attributes and behavior as a single entity—allows us to build on our understanding of the natural world as we create software.
encapsulation
Constructors have the same name as the ____. 1. data members 2. member methods 3. class 4. package
3. class
int[] num = new int[100]; for (int i = 0; i < 50; i++) num[i] = i; num[5] = 10; num[55] = 100; What is the data type of the array above? 1. list 2. num 3. char 4. int
4. int
When you create your own new, user-defined types, there are really three different strategies you can use. Which of these is not one of those strategies? 1. defining a component from scratch, inheriting only the methods in the Object class 2. extending an existing component by adding new features 3. defining a component from scratch, inheriting no methods whatsoever 4. combining simpler components to create a new component
3. defining a component from scratch, inheriting no methods whatsoever
To call a superclass method from a subclass method, when both methods have the same name and signature, prefix the call to the superclass method with: 1. super 2. the name of the superclass (that is, Person, if that is the name of the superclass) 3. no prefix is necessary. You cannot call an overridden superclass method from the subclass method you've overridden. 4. this 5. base
1. super
public class Illustrate { private int x; private int y; public Illustrate() { x = 1; y = 2; } public Illustrate(int a) { x = a; } public void print() { System.out.println("x = " + x + ", y = " + y); } public void incrementY() { y++; } } Which of the following statements would you use to declare a new reference variable of the type Illustrate and instantiate the object with a value of 9 for the variable x? 1. Illustrate.illObject(9); 2. Illustrate illObject = new Illustrate(9); 3. Illustrate illObject(9); 4. illObject = Illustrate(9);
2. Illustrate illObject = new Illustrate(9);
___________________ is one of the primary mechanisms that we use to understand the natural world around us. Starting as infants we begin to recognize the difference between categories like food, toys, pets, and people. As we mature, we learn to divide these general categories or classes into subcategories like siblings and parents, vegetables and dessert. 1. classification 2. encapsulation 3. specialization 4. generalization
1. classification
The toString() method is automatically inherited from the __________________ class. 1. GObject 2. Object 3. java.lang 4. Root 5. Component
2. Object
What does the following loop do? int[] a = {6, 1, 9, 5, 12, 3}; int x = 0; for (int e : a) x += a; 1. Sums all the elements in a. 2. Will not compile (syntax error) 3. Finds the largest value in e. 4. Counts each element in a.
2. Will not compile (syntax error)
Assume that you have an ArrayList variable named a containing 4 elements, and an object named element that is the correct type to be stored in the ArrayList. Which of these statements assigns the last object in the collection to the variable element? 1. element = a[3]; 2. element = a[4]; 3. element = a.get(3); 4. a.get(3, element); 5. a.get(element, e);
3. element = a.get(3);
Section 37
(50 cards)
Use a remainder assignment operator to make c=3 when c=12
c%=9
Natural logarithm of a number?
Math.log()
What is sentinel controlled repetition?
When a loop is controlled by a sentinel value such as s!=3
When are the &, |, and ^ operators also bitwise operators?
when they're applied to integral operands
Round to smallest integer not less than number input(round number up)?
Math.ceil(#) Ex: ceil(9.2)=10
What are LOGICAL operators of Java? What does each one mean?
&& which is the Conditional AND operator-Statement is only true when both conditions are true (true && true) | |-the Conditional OR operator-Statement is only true as long as one of the statements are true(true || false) (true || true) (false || true) & which is the Boolean logical AND operator-... | which is the boolean logical inclusive OR operator-... ^- the boolean logical exclusive operator OR -Statement is only true when one condition is false and one is true (true ^ false) (false ^ true) ! which is the logical NOT operator- does the same function as != except this operator comes before the statement (!(statement))
How do you set up a switch method?
switch () Case #: (statement)(++acount) (break;) Case #: (statement) (break;) Case #: (statement)
Cosine?
Math.cos(#)
Prints a formatted string
printf
Make g conditional upon some statement
?:g
Use a subtraction assignment operator to make c=5 when c=7
c-=2
What is the class before the keyword, extends called? the one after?
subclass Superclass
What is a local variable?
can be used only in the method that declares it from point of declaration to end of method
Is the break statement on a switch needed for the last statement/default?
no
How do you test a diagram as to whether it is structured?
apply the rules backwards to reduce it to the simplest activity diagram.
Use a multiplication assignment operator to make c=8 when c=2
c*=4
Round to the largest integer not greater than number input?(round down)
Math.floor(#)
absolute value?
Math.abs(#)
What is the format for a, for, statement?
for (initialization;loopContinuationCondition;increment) statement (Initialization=declared variable and value) (loopContinuationCondition=i<5) (Statement=(println)
Pi
Math.PI
Make c not equal to b
c!=b
Prints to the current line(single function)
What is the sentinel value?
a value to be typed in to stop a loop
Square Root
Math.sqrt
What is program control?
Specifying the order in which statements(actions) execute in a program.
When can you not use a (while) statement instead of a (for) statement? How does it differ?
When the (while) statement has an increment after a continue statement, it is different than a (for) statement. they differ in execution.
What is counter controlled repetition?
When a loop is controlled by a counter such as x>3
How do you put e to a power? Ex: e^x
Math.exp(x)
What is the scope of a variable?
It defines where the variable can be used in a program.
What does a UML activity diagram do?
It models the workflow(also called the activity) of a portion of a software system.
What is the end of file indicator for windows? and for UNIX/Linux/Mac OS X?
<Ctrl> z <Ctrl> d
Why is the method "main" Declared static?
This allows the JVM to invoke "main" without creating an object of the class
What is control statement stacking?
When a number of control statements are placed one after another in a sequence within a program.
Use a division assignment operator to make c=4 when c=8
c/=2
What does c++ do? and what is it called? and c--?
prints result then adds 1. Postfix increment prints result then subtracts 1. Postfix decrement
Power or Exponent
Math.pow(x,y)
What does ++c do? and what is it called? And --c?
adds 1 to c then prints out result. Prefix increment subtracts 1 from c then prints out result. Prefix decrement
Random Number
Math.random()
In a (for), (while), or, (do...while) statement what does the break statement do? and a continue statement?
It breaks out of the loop using an if statement ' Ex: if (count==5) break; 1 2 3 4 It skips the number defined Ex: if (count==5) continue; 1 2 3 4 6 7 8 9 10
Where should the default statement be placed within a switch statement?
last
What are the rules for forming a structured program?
1. Begin with the simplest activity diagram { @->(action state)->(@)} 2. Any action state can be replaced by two action states in sequence. (A.K.A stacking rule) 3. Any action state can be replaced by any control statement. (A.K.A the nesting rule) 4. Rules 2 and 3 can be applied as often as you like and in any order.
Use an Addition assignment operator to make c=5 when c=2
c += 3
What are the three types of class variables?
Public, Allows them to be used in your own classes Static, allows them to be accessed vie that class name and a dot (.) separator Final, indicates that they are constants--- their values cannot change after they are initiated.
What are the 3 selection statements? And the 3 Repetition Statements?
If, If...else, and Switch While, do...while, and for
What is the format for a, while, statement?
initialization; while (loopContinuationCondition) { Statement increment; } ex: pg 91
What does the keyword, Extends, do?
indicates that the class before the keyword is an enhanced type of the class following the keyword meaning that the methods and data from the class before the keyword are added on to the methods of the class after the keyword.
What does it mean in an && operator statement to have a dependent condition?
The dependent condition is the one that gets tested first and must come after the && operator
How do you call a method from a class?
ClassName.methodName(arguments)
What is the definition of an algorithm?
A procedure for solving a problem in terms of: The actions to execute and the order in which these actions execute
Prints to a new line and flushes the buffer.
println
Section 38
(50 cards)
Demo
The filename of the source file must be the same as the class name (###case sensitive###) java MyFirstApp (###not MyFirstApp.class###)
What are the three ways to call a method?
Using the method name itself within the same class Using the object's variable name followed by a dot (.) and the method name to call a non static method of the object. Using the class name and a dot(.) to call a static method of a class: Math.pow
Bit Permutations
Each additional bit doubles the number of possible permutations Each permutation can represent a particular item N bits can represent up to 2^N unique items, so to represent 50 states, you would need 6 bits: 1 bit: 2^1 = 2 items 2 bits: 2^2 = 4 items 3 bits: 2^3 = 8 items 4 bits: 2^4 = 16 items 5 bits: 2^5 = 32 items Five bits wouldn't be enough, because 2^5 is 32. Six bits would give us 64 permutations, and some wouldn't be used.
What compilation error should you be aware of when using arguments and parameters?
If the amount of arguments(maximum(a, b, c) does not match the amount of parameters(maximum( double x, double, y, double z, double d) this creates a compilation error
Program Development
The mechanics of developing a program include several activities: -writing the program (source code) in a specific programming language (such as Java) using an editor -translating the program (through a compiler or an interpreter) into a form that the computer can execute -investigating and fixing various types of errors that can occur using a debugger Integrated development environments, such as Eclipse, can be used to help with all parts of this process
Java Reserved Words
You cannot use use reserved words for your own names (Java identifiers)
larger value of x and y?
Math.max(x,y)
Java standard class library (Java API)
a set of predefined classes and methods that other programmers have already written for us System, out, println and String are not Java reseved words, they are part of the API
Computer Processing
Java Program Structure
In the Java programming language: -A program is made up of one or more classes defined in source files (with the .java extension) -A class contains one or more methods -A method contains program statements
What Java Can Do
Science and Technology -Popular machine learning libraries are written in Java: Mahout, Weka -Has been adopted in many distributed environments (Hadoop) Web development -Most web backend are developed using Java Mobile application development -Android development platform Graph user interface (GUI) design: menu, windows, ...
Smaller value of x and y?
Math.min(x,y)
Another Example
Output A quote by Abraham Lincoln: Whatever you are, be a good one.
Sine?
Math.sin(#)
Problem Solving
The purpose of writing a program is to solve a problem The key to designing a solution is breaking it down into manageable pieces An object-oriented approach lends itself to this kind of solution decomposition We will dissect our solutions into pieces called classes (.java files) We design separate parts (classes) and then integrate them
How is the filename of a public class formatted?
Filename.java
Binary Numbers
Information is stored in memory in digital form using the binary number system A single binary digit (0 or 1) is called a bit A single bit can represent two possible states, like a light bulb that is either on (1) or off (0) Permutations of bits are used to store values Devices that store and move information are cheaper and more reliable if they have to represent only two states
What must come after a declared class?
A pair of brackets that contain the method(s) between them
Object-Oriented Programming
Classes are the building blocks of a Java program; a blueprint for which individual objects can be created Each class is an abstraction of similar objects in the real world E.g., dogs, songs, houses, etc Once a class is established, multiple objects can be created from the same class. Each object is an instance (a concrete entity) of a corresponding class E.g., a particular dog, a particular song, a particular house, etc
Java Program Structure Continued
What does a scanner do for a program?
Enables the program to read data (ints, dbls, char etc.)
What are the three ways to return control to the statement that calls a method?
Let the second bracket of the called method return control use return; to return control use return (expression); to return a variable as well as control
What is the definition of the Java class Library A.K.A Java A.P.I(Java Application Programming Interface)?
This is the list of preprocessed classes, packages, and methods that you may use at your disposal such as the Math....() methods.
When naming variables, what is camel case?
When the words following the first word having a captial first letter: alphaQuadron
Tangent?
Math.tan(x)
How Java Works
The compiled bytecode is not the machine language for traditional CPU and is platform-independent javac and java (two command-line tools) can be found in the directory of bin/ of Java Development Kit (JDK) you will install
What do you attach to a method if it requires more information?
A parameter that represents additional information: Method: public static double _maximum_(Parameters(double x, double y, double z))
What syntax error should you be aware of when entering parameters for a method?
If you set two variables as the same type, but don't use two type indentifiers then it creates a syntax error. You should always use a type identifier for each variable: double x, y = NO double x, double y = YES
How to Execute Programs
Each type of CPU executes only a particular machine language A program must be translated into machine language before it can be executed Compiler: translates only once Most high-level languages: C, C++, etc. Interpreter: translates every time you run programs Most scripting languages: python, perl, etc. The Java approach is somewhat different. It combines the use of a compiler and an interpreter.
Class=Blueprint
Just like how one blueprint (class) is used to create several similar, but different houses (objects)
When should camel case be used?
When defining the type of an input: int greatNumber, dbl, char etc. When naming a method or class: class( QuadraticFormula), Method(equationProcessor)
What are the local variables/parameters of a method stack known as?
Stack frames/activation records
What type of structure are stacks usually known as?
LIFO structures (Last-in, First-out) which means that the last item pushed onto the stack is the first to be taken off.
Basic Program Execution
If there are errors the compiler will complain and not let us go onto the next step. We must trace back to the source and fix the problem in our code. It's important to understand why you're getting errors. LOOK IT UP debuggers can help you find errors in your code
Language Levels
There are four programming language levels. Each type of CPU has its own specific machine language The other levels were created to make it easier for a human being to read and write programs
What does JVM stand for?
Java Virtual Machine
White Space
Spaces, new lines, and tabs are called white space White space is used to separate words and symbols in a program Extra white space is ignored by the computer A valid Java program can be formatted many ways Programs should be formatted to enhance readability, using consistent indentation
Where must all import declarations appear?
Before the first class declaration
Categorize each of the following situations as a compile-time error, run-time error, or logical error.
A. Multiplying two numbers why you mean to add them. logical error B. Dividing by zero run-time error C. Forgetting a semicolon at the end of a programming statement compile-time error D. Spelling a word incorrectly in the output logical error E. Producing inaccurate results logical error F. Typing { when you should have typed a ( compile-time error
Comments
There are two forms of comments, which should be included to explain the purpose of the program and describe processing steps They do not affect how a program works
Logical or semantic errors (Error Type 3/3)
a program may run, but produce incorrect results, perhaps using an incorrect formula
How to Write
Syntax
The rules of a language that define how we can combine symbols, reserved words, and identifiers to make a valid program -Identifiers cannot begin with a digit -Curly braces are used to begin and end classes and methods
Each class declares Instance Variables and Methods (Unique Values for the Objects in the Class)
Instance variables: descriptive characteristics or states of the object Ex. students are an instance variable of a class Methods: behaviors of the object (the actual doing thing of each instance variable) Ex. doing homework or taking exams are the methods of the instance variable "student"
The Main Method
In Java, everything goes in a class (object-oriented) After you compile the source code (.java) into a bytecode or class file (.class), you can run the class using the Java Virtual Machine (JVM) When the JVM loads the class file, it starts looking for a special method called main, and then keeps running until the code in main is finished
Run-time errors (Error Type 2/3)
a problem can occur during program execution, such as trying to divide by zero, which causes a program to terminate abnormally
Semantics
The semantics of a program statement define what that statement means (its purpose or role in a program) A program that is syntactically correct is not necessarily logically (semantically) correct A program will always do what we tell it to do, not what we meant to tell it to do
Who will do the Java compiling?
Javac is the complier, not Eclipse. Eclipse is the software that runs javac. The Java virtual machine (the executable response) is called java For one class... what is the mechanism that allows us to run it? Answer: we are using an existing object
Compile-time or syntax errors (Error Type 1/3)
the compiler will find syntax errors and other basic problems
Java Identifiers
Identifiers are the "words" in a program -Name of a class: Dog, MyFirstApp, and Lincoln -Name of a method: bark and main -Name of a variable: args (later in this course) Rules: A Java identifier can be made up of -letters -digits (cannot begin with a digit) -the underscore character ( _ ), -the dollar sign ($) Java is case sensitive: -Total, total, and TOTAL are different identifiers By convention, programmers use different case styles for different types of identifiers, such as title case for simple class names - Lincoln camel case for compound words - MyFirstApp upper case for constant variables - MAXIMUM
Section 39
(50 cards)
Expressions
Order of operations is applied: Multiplication, division, and remainder are evaluated before addition, subtraction, and string concatenation Arithmetic operators with the same precedence are evaluated from left to right, but parentheses can be used to force the evaluation order Assignment operator is last
String Concatenation
The string concatenation operator (the plus symbol) is used to append one string to the end of another. In the case of a long string, you must use concatenation to break it across multiple lines.
Numeric Primitive Types
Data Conversion
Sometimes it is convenient to covert data from one type to another For example in a particular situation we may want to treat an integer as a floating point value These conversions do not change the type of a variable or the value that's stored in it - they only convert a value as part of a computation Widening conversions are safest because they tend to go from a small data type to a larger one (such as a short to an int) Narrowing conversions can lose information because they tend to go from a large data type to a smaller one (such as an int to a short) In Java, data conversions can occur in three ways: -assignment conversion -promotion -casting
Packages
For purposes of accessing them, classes in the Java API are grouped into packages
Casting Conversion
Both widening and narrowing conversions can be accomplished by explicitly casting a value To cast, the type is put in parentheses in front of the value being converted double money = 84.69; int dollars =(int) money; // dollars = 84
Dog Example
Will see the syntax of creating objects (with unique instance variables) and then calling methods from the class later The computer will read this and decide which bark to use Just think of it like you name something (object) and give it variables and then it takes the information and outputs something (method)
New
new is a Java reserved word that triggers all kinds of actions. goes back to the heap (free memory space), reserves a chunk of the memory for the real value (address) Java actually invokes a special method called constructor, provided by programmer, which does the proper initialization new is the only thing that produces a reference type
Constant
A constant is an identifier that is similar to a variable except that it holds the same value during its entire existence As the name implies, it is constant and cannot be changed The compiler will issue an error if you try to change the value of a constant In Java, we use the final modifier to declare a constant final int NUM_SEASONS = 4; whenever you use the reference variable, NUM_SEASONS, it will equal 4 Constants are useful for three important reasons First, they give meaning to otherwise unclear literal values Example: MAX_LOAD means more than the literal 250 Second, they facilitate program maintenance If a constant is used in multiple places, its value need only be set in one place Third, they formally establish that a value should not change, avoiding inadvertent errors by other programmers
The + Operator
The + operator is also used for arithmetic addition The function that it performs depends on the type of the information on which it operates If both operands are strings, or if one is a string and one is a number, it performs string concatenation If both operands are numeric, then it adds them The + operator is evaluated left to right, but parentheses can be used to force the order If both of its operands are strings, or if one is a string and one is a number, it performs string concatenation. But if both operands are numeric, it performs addition.
The string class
Every character string (enclosed in double quotation characters) is an object in Java defined by the String class Every character string can be used to create a string object Because strings are so common, we don't have to use the new operator to create a String object (special syntax that works only for strings) String name1 = "Steve Jobs"; It is equivalent to String name1 = new String("Steve Jobs");
Java Class Libraries
A class library is a collection of classes to facilitate programmers when developing programs The Java standard class library is part of any Java development environment (available after installing JDK) The Java standard class library is sometimes referred to as the Java API (Application Programming Interface) Its classes are not part of the Java language per se, but we rely heavily on them Various classes we've already used (System, String) are part of the Java API
Creating Objects: Reference Variable
Dog is the data type. That's what reference allows us to do. It makes it user friendly by letting us make our own data types. We tell the computer to set aside a chunk of space for the data type, Dog. The size of the chunk will be big enough to hold an address, because reference is indirect. No actual amount will go in like with primitive datatypes where we know the amount of storage being used. It will just leave space (a placeholder)
More About System.out
The System.out object represents a destination (the monitor screen) to which we can send an output The System.out object provides another service in addition to println The print method is similar to the println method, except that it does not advance to the next line Therefore anything printed after a print statement will appear on the same line
Example of the Dot Operator in Use
System.out.println revisited There is a class called System, which contains an object reference variable out as one of its instance variables The object referenced by out has a method called println
Example of setting a reference to null
Charachter Strings
A string literal is represented by putting double quotes around the text examples: "I Rule!" "The World" "123 Main Street" "X" "" literal is an explicit data (constant) value used in a program Every character string is an object in Java, defined by the String class Every string literal represents a String object A string literal can contain any valid characters, including numeric digits, punctuation, and other special characters.
Example of Declaring a Variable and then changing it with the assignment operator
Scanner Class
It is often useful to design interactive programs that read input data from the user The Scanner class provides convenient methods for reading input values of various types A scanner object can be set up to read input values from various sources including the user typing values on the keyboard The following line creates a Scanner object that reads from the keyboard: Scanner scan = new Scanner(System.in); Once created, the Scanner object can be used to invoke various input methods, such as: String answer = scan.nextLine(); The nextLine method reads all of the input until the end of the line is found, and return it as one String
Creating objects
How to create objects once a class is established? How to "hold" those created objects? For primitive types (such as byte, int, double), we use a variable of that particular type as a cup to store its actual value BUT Objects are different There is actually no such thing as an object variable, because we can't put a number on it. There is only an object reference variable An object reference variable holds something like a pointer or an address that represents a way to get to the object. When you copy a string object, you're really copying it's address.
Garbage Collection
When an object no longer has valid references to it, it can no longer be accessed by the program The object is useless and therefore is called garbage Java performs automatic garbage collection periodically, returning and object's memory to the system for use In other languages the programmer is responsible for performing garbage collection
Three step of object declaration, creation and assignment
Reference Assignment
Once a String object has been created, neither its value nor its length can be changed (it's immutable) However, its reference variables can change
String mutation example to put into Java
Even though we can't change String value or length, several methods of the String class return new String objects that are modified versions of the original
Increment and Decrement
Write four different program statements that increment the value of an integer variable total. Count ++ Count = Count + 1 Count = ++ Count Count += 1
Classes contain methods that determine the behaviors of objects
Two common usages of methods: -change the values of the instance variables, e.g., making deposits to a bank account object should change its current balance -methods can behave differently based on the values of the instance variables, e.g., playing a "Darkstar" song object and a "My Way" song object should be somehow different...
What output is produced by the following?
X: 25 Y: 65 Z: 30050
Some String Methods
String name1 = new String("Steve Jobs"); int numChars = name1.length(); char c = name1.charAt(2); String name2 = name1.replace('e', 'x'); String name3 = name1.substring(1, 7); String name5 = name1.toLowerCase();
Example of three references and two object
d and c are aliases, because they both reference the same book
What are the results of the following expressions?
numChars = name1.length();
// returns the number of characters name1 contains
Charachters
A char variable stores a single character (16 bits)
Character literals are delimited by single quotes:
'a' 'X' '7' '$' ',' '
'
Example of declarations with initialization:
char topGrade = 'A';
char terminator = ';', separator = ' ';
Note the difference between a primitive character variable, which holds only one character (enclosed by single quotes), and a String object, which can hold multiple characters (enclosed by double quotes)
Promotion Conversion
Promotion happens automatically when operators in expressions convert their operands Example: int count = 12; double sum = 490.27; double result = sum / count; The value of count is converted to a floating point value to perform the division calculation
Boolean
A boolean value represents a true or false condition (1 bit) The reserved words true and false are the only valid values for a boolean type boolean done = false; A boolean variable are usually used to indicate whether a particular condition is true, such as whether the size of a dog is greater than 60
The Import Statement
Why didn't we import the System or String classes explicitly in earlier programs? All classes of the java.lang package are imported automatically into all programs It's as if all programs contain the following line: import java.lang.*; The Scanner class, on the other hand, is part of the java.util package, and therefore must be imported
The right and left hand sides of an assignment statement can contain the same variable
Example of two references and two objects
Declaration with Initialization
An assignment statement changes the value of a variable The assignment operator is the = sign If you wrote keys = 80; The 88 would be overwritten You should assign a value to a variable that is consistent with the variable's declared type
Inheritance
One class can be used to derive another via inheritance Classes can be organized into hierarchies
Variables
A variable is a name for a location (a chunk) in memory that holds a value (a fixed type of data) Variables come in two flavors: primitive and reference primitive: integer (such as 1, 2,...), floating point number (such as 1.2, 3.5, etc.), boolean (true or false) and char (such as 'A', 'B',...) reference: "address" of an object that resides in memory A variable declaration specifies the variable's name and the type of information that it will hold
Objects and the Heap
The number of objects that a program creates is not known until the program is actually executed Java uses a memory area called the heap to handle this situation, which is a dynamic pool of memory on which objects that are created are stored When an object is created, it is placed in the heap -Two or more references that refer to the same object are called aliases of each other -One object can be accessed using multiple reference variables -Changing an object through one reference changes it for all of its aliases, because there is really only one object
Primitive Types
There are eight Primitive data types in Java Four of them represent integers • byte, short, int, long Two of them represent floating point numbers • float, double One of them represents a single character • char And one of them represents boolean values • boolean
String Indexes
It is occasionally helpful to refer to a particular character within a string This can be done by specifying the character's numeric index The index begins at zero in each string In the string "Hello" H is at the index 0 and o is at the index 4
String mutation example Output
The Dot Operator
The dot operator (.) gives you access to an object's instance variables (states) and methods (behaviors) It says: use the Dog object referenced by the variable myDog to invoke the bark() method Think of a Dog reference variable as a Dog remote control When you use the dot operator on an object reference variable, it is like pressing a button on the remote control for that object
Assignment Conversion
Assignment conversion occurs when a value of one type is assigned to a variable of another Only widening conversions can happen via assignment
Quick Check: Who am I?
Escape Sequence
What if we wanted to print the quote character? The following line would confuse the compiler because it would interpret the second quote as the end of the string System.out.println ("I said "Hello" to you."); An escape sequence is a series of characters that represents a special character An escape sequence begins with a backslash character (\) System.out.println ("I said \"Hello\" to you.");
Assignment Operators
Often we perform an operation on a variable, and then store the result back into that variable Java provides assignment operators to simplify that process For example, the statement num += count; is equivalent to num = num + count; Other assignment operators: -=, *=, /=, %=
Division and Remainder
Section 40
(47 cards)